3
The Growth of Plants

Leonardo’s science is a science of living forms that are continually shaped by underlying processes, whether he studied the rocks and sediments of the Earth, shaped by water, or the organic forms of plants, animals, and the human body, shaped by their metabolism. Invariably, he would begin with the outward appearance of these living forms and then proceed to investigate their intrinsic nature. Thus, at the core of his botanical studies, we find the two grand themes that appear again and again in other branches of his science—nature’s organic forms and the patterns of metabolism and growth underlying them.

Leonardo’s outstanding work in botany, as well as his original contributions to landscape and garden design, are discussed in great detail in the magnificent volume Leonardo da Vinci on Plants and Gardens, by botanist William Emboden.¹ This chapter is greatly indebted to Emboden’s analysis.²

Unlike most of his other scientific studies, Leonardo’s work in botany began relatively late in his life (see Chronology, p. 326). During the earlier years, his drawings of plants and trees were made mainly as studies for paintings. Notes on plants and landscape, often dealing with colors and light in addition to botanical accuracy, appear in his manuscripts most frequently after 1500, when he was forty-eight years old. The skill in his botanical drawings reached its culmination around 1508–10, and it was only after 1510, when Leonardo was in his sixties, that his botanical texts turned into purely scientific inquiries unrelated to paintings.

Plants were frequently used by Renaissance painters to decorate the geometrical and abstract spaces that were typical of the paintings of the time, especially in the Florentine school. These plants were usually arranged in formal decorative motifs. Some were rendered accurately, while others were purely imaginary. Botticelli’s celebrated Primavera, for example, pictures a complex allegory in a garden setting with a glorious abundance of flowers. According to Emboden, “thirty of the forty plant species are identifiable and some are very well figured. Others are imaginary and seem included for mere decorative value.”³

FACING “Star of Bethlehem,” c. 1508 (detail, see plate 6).

Beyond decoration, many plants in Renaissance art served another important purpose, especially in religious paintings. They were often associated with religious stories well known to the public, and thus had the “iconographic” function of conveying meaning through symbolic imagery. Leonardo exploited these additional layers of meaning in many of his paintings while depicting the plants that embodied the appropriate symbols with high botanical accuracy and masterful renderings of light and shade.

In addition, Leonardo was careful to represent the plants within their proper habitats and with seasonal accuracy. He did this to such an extent that a modern botanist today can recognize, for example, the setting of the Virgin of the Rocks as a Tuscan scene between March and April.⁴ All these characteristics are what make the plants in Leonardo’s masterpieces so unique. In the words of Emboden:

Free from formal definitions that characterize Mantegna, Ghirlandaio, Perugino, and Botticelli in their use of plant motifs, Leonardo introduced an enormous vitality into his plant figurations. Beyond the iconographic use of plants, [he] considered the ecological context, seasonal correctness, and careful botanical depiction …. Leonardo’s forceful environments seem charged and infused with an organic unity.⁵

Spiral Movement

When Leonardo investigated “all the forms of nature” in various branches of his science, he always looked for the processes and patterns of organization they had in common. One particular pattern that fascinated him throughout his life was that of spiral movement (moto elico). As mentioned earlier, Leonardo saw the spiral form as an archetypal code for the ever-changing and yet stable nature of living forms (see p. 62). He observed and drew it repeatedly in swirling vortices of water and air, in growth patterns of plants and animals, in curling locks, and in human movements and gestures.

FIG. 3-1. Andrea del Verrocchio and Leonardo da Vinci, “Study for the Giostra,” 1475. Uffizi Gallery, Florence, Gabinetto Disegni e Stampe.

In his botanical drawings, Leonardo sometimes pictured stylized spiraling foliage to express his strong sense of the dynamic nature of organic forms. He adopted this habit very early on; in fact, it is evident in one of the very first botanical drawings from his hand that has come down to us.

In the mid-1470s, when Leonardo had just been certified as a master painter but was still working in Verrocchio’s bottega (workshop), his teacher collaborated with him on the preliminary design for a triangular tournament standard made of cloth, showing a winged Cupid and a recumbent nymph (fig. 3-1). The standard had been commissioned by the Medici family for a lavish pageant preceding a jousting tournament (giostra) in 1475, and the drawing, now in the Uffizi Gallery in Florence, is known as “Study for the Giostra.” Its origin and authorship were long controversial, but scholars now agree that Verrocchio made a preliminary sketch in black chalk, barely visible in the original, and then let Leonardo fix and elaborate it in pen and ink.⁶ Being well aware of his pupil’s special talent for rendering natural forms, Verrocchio also left the drawing of the landscape entirely to Leonardo, who added the plants and the rocky ledge on which the nymph reclines.

The long-stemmed plants from which Cupid emerges are rendered so accurately that they could be identified by botanists as a species of tall grass known as broomcorn millet (Panicum miliaceum). However, Leonardo chose to give the lower leaves a swirling, spiraling movement that is not characteristic of that plant. This spiral form of the leaves was an original creation of the young Leonardo that had no precedent in Renaissance art.⁷ It seems that from the beginning of his career, Leonardo must have viewed patterns of plant growth as manifestations of a more general pattern embodied in various forms of organic life.

Ten years later, the stylized spiraling foliage appeared again in Leonardo’s first finished masterpiece, the Virgin of the Rocks (plate 8), which contains an entire ecosystem of exquisitely rendered plants. In the lower left corner, Leonardo painted a tall species of iris known as yellow flag iris (Iris pseudoacorus). He accurately pictured its characteristic flowers and flat, sword-like leaves. But whereas in nature its leaves emerge from the ground in a fan-like arrangement in a single plane, Leonardo introduced a spiral movement into the foliage that is very similar to that of the broomcorn millet he drew in his youth. In this masterpiece, now in the Louvre, the spiral form of the plant’s lower leaves is much more pronounced, conveying not only a strong sense of growth and vitality but also an impression of compelling elegance.

Leonardo’s drawings of spiraling foliage reached a climax around 1506–8 in his studies for Leda and the Swan, in which the artist’s central theme was the mystery of life’s inherent procreative power (see p. 320). A preliminary study, now in the Rotterdam Collection, shows us a sensuous female nude kneeling in a moist swamp and turning toward the swan at her side with a gesture of great tenderness (fig. 3-2). The erotically charged composition is heightened by the phallic reed mace (Typha latifolia; also called bulrush or cattail) silhouetted against the sky and by the swirling grasses at her feet. The spiral movement of Leda’s body is repeated in the swan and in the spiraling foliage surrounding them—all symbolizing the abundance of life’s generative forces.*

FIG. 3-2. Study for Leda and the Swan, c. 1505–10. Museum Boijmans Van Beuningen, Rotterdam.

Leonardo made several studies of individual wetland plants in preparation for Leda and the Swan, including his celebrated drawing of the Star of Bethlehem (Ornithogalum umbellatum, plate 6), the species botanists have identified as the grasses at the feet of the kneeling Leda in the Rotterdam study. The foliage in this highly stylized drawing forms the most exaggerated spirals. Indeed, the whole composition is strikingly reminiscent of a water vortex, another of Leonardo’s archetypal living forms. During the same period, Leonardo also produced a study for the head of Leda (fig. 3-3) in which the same swirling movement appears in her hair—one more manifestation of the spiral as symbol of nature’s fecundity and procreative power.

“Many flowers portrayed from nature”

Plants and trees play important roles in nearly all of Leonardo’s paintings. They have symbolic meanings and convey metaphoric messages while displaying the artist’s profound knowledge of botanical forms and their underlying processes. Flowers seem to have been Leonardo’s first subject when he showed great talent for drawing as a young boy in his native Vinci. In his Notebooks many years later, he listed “many flowers portrayed from nature” among the works he had produced in his early youth.⁸

Leonardo’s sophisticated botanical and ecological understanding is fully displayed in his early masterpiece, the Virgin of the Rocks (plate 8). As mentioned earlier, the painting has been called “a geological tour de force” because of the artist’s astonishingly accurate representation of complex geological formations (see p. 77). It might be called a botanical tour de force with equal justification. The luxuriant plants filling the rocky grotto are not scattered about the painting in a decorative pattern but are shown to grow only in places where weathered sandstone has decomposed sufficiently to allow their roots to take hold (see p. 78). Only species appropriate to the moist environment of the grotto are portrayed, each in a specific habitat and a seasonally accurate stage of development.⁹

Within those botanical and ecological constraints, Leonardo selected specific plants that evoked for his contemporaries multiple layers of subtle symbolic meanings associated with the religious themes of his composition.¹⁰ Behind the Virgin’s left shoulder is a graceful columbine (Aquilegia vulgaris). Its Latin name is derived from aquila (eagle), as the flowers were thought to resemble an eagle’s talon. In antiquity, the plant was also known as “lion’s herb” and its common name, columbine, alludes to the resemblance of the flower to a cluster of doves. To the Renaissance mind, these associations were rich in religious symbolism. The eagle and the lion were the symbols of the evangelists John and Mark, the dove personified the Holy Spirit, and the columbine’s tripartite leaves were a perfect symbol of the Trinity.

Just above the Virgin’s left hand one can barely see a cluster of tiny whirls, formed by the leaves of a plant known as Our Lady’s bedstraw (Galium verum). According to legend, Joseph used the dried straw of this plant in the manger to make a bed for Mary, and its white blossoms turned to radiant gold when Jesus was born.

FIG. 3-3. Study for the head of Leda (detail), c. 1505–10. Windsor Collection, Figure Studies, Profiles, and Caricatures, RL 12516 (detail).

The rosette of leaves above the knee of the Christ Child has been identified by Emboden as belonging to the primrose Primula vulgaris, which was considered an emblem of virtue because of its pure white flowers. Emboden points out that the purity of Christ was usually represented by a white rose, but that Leonardo chose the white primrose instead because a rose would have been inappropriate for the given setting and season.

Several plants in the painting allude to various stages in the Passion of Christ. The palm leaves above the infant Saint John, identified with the genus Raphis, were an ancient symbol of immortality and evidently are meant here to herald Christ’s entry into Jerusalem, just as Saint John would herald Christ as the Messiah. The three clusters of leaves behind the infant Saint John could belong to several plant species. However, in view of the implied time of year, they have been identified by Emboden as representing the anemone known, because of its trifoliate leaves, as herb trinity (Anemone hepatica). A related small cluster of anemones (Anemone hortensis) can be seen under the seated Christ Child. The anemone represented the blood drops of Christ and was said to have blossomed under the cross on Calvary when the blood fell from Christ’s wounds. Finally, the Resurrection of Christ is symbolized by the leaves of bear’s breech (Acanthus mollis) between the right knee and left heel of Saint John. As Emboden explains, it was an Italian tradition to plant bear’s breech over graves, where it came to symbolize the Resurrection because it dies back to the ground in autumn and reemerges rapidly with a wealth of green foliage in spring.

The elegant iris in the lower left corner of the painting, with its striking spiraling leaves, has already been mentioned (see p. 100). Emboden points out that this is not the species Iris florentina, which Leonardo often portrayed in his drawings, but instead the ecologically appropriate wetland species Iris pseudoacorus.¹¹

Many more plant species are portrayed in the Virgin of the Rocks, all chosen for particular symbolic virtues. They include St. John’s Wort (Hypericum perforatum), the plant consecrated to Saint John and believed to have protective powers; a cyclamen (Cyclamen purpurascens), which symbolizes love and devotion because of its heart-shaped leaves; several species of ferns, believed to be benevolent repositories of souls; and branches of oak (Quercus robur), which embodies a host of iconographic virtues.

As mentioned earlier, there are two versions of the Virgin of the Rocks, one now in the Louvre and the other, painted several years later, in the National Gallery in London. It is widely believed that Leonardo let his fellow painter Ambrogio de Predis execute large portions of the London version. We have seen that this seems to be confirmed by a comparison of the geological details in both paintings (see p. 80).

Emboden came to a similar conclusion after comparing details of the plants in the two paintings. He points out that there are fewer plant species in the London version and shows that many of them are rendered inaccurately and without the sophistication displayed in the Louvre version. This leads Emboden to the conclusion that in the London version, “most certainly the plant life is not from the hand of Leonardo…. It is impossible to believe that the same painter who, in the Paris version of the same painting, took such great care to render plants with seasonal and ecological accuracy, not to mention iconography, produced the precarious landscape with simplistic conventions of botanical presentation.”¹²

Botany for Painters

During his earlier years, Leonardo drew individual plants mainly as studies for paintings. Later on, he also jotted down in his Notebooks instructions for painters on how to render the effects of light and shade and the diversity of colors he observed in nature. These notes on how to paint plants and trees became increasingly frequent after 1500. They are collected in Jean Paul Richter’s classic selection of Leonardo’s writings in a section titled “Botany for Painters.”¹³ The excerpts collected in this section, filling almost forty pages, contain detailed descriptions of subtle color variations and of the effects of light and shade on various parts of trees and plants. In the Codex Arundel, for example, Leonardo noted:

The trees in a landscape are of various kinds of green, inasmuch as some verge toward blackness, as firs, pines, cypresses, laurels, box and the like. Some tend toward yellow, such as walnuts and pears, vines and verdure. Some are both yellowish and dark, as chestnuts and common oak.¹⁴

And in Manuscript G:

Young plants have more transparent leaves and a more lustrous bark than old ones; and particularly the walnut is lighter in May than in September.¹⁵

Leonardo was indeed a master in rendering the appearance of trees under various light conditions. In the words of Emboden, Leonardo’s trees manifest “an omnipresent mystical quality imparted by the juxtaposition of light and shadow.”¹⁶ A sheet in the Windsor Collection (fig. 3-4) contains a particularly elegant example of a single tree drawn in red chalk, in which these subtle optical effects are superbly displayed. Leonardo’s accompanying note reads:

That part of a tree which stands out against shadow is all of one tone, and where the trees or branches are thickest, there it is darker because light has less of an impression there. But where the branches are against other branches, there the luminous parts show themselves brighter, and the leaves shine as the sun illuminates them.¹⁷

Only a very few of Leonardo’s early studies of individual plants have come down to us. One of the finest, and perhaps the most famous, is his Madonna lily (Lilium candidum) from the early 1470s (fig. 3-5). This is the same species as the lily held by the angel in Leonardo’s Uffizi Annunciation, but the two differ in the arrangement of their flowers, buds, and leaves. The study is an impressive testimony to Leonardo’s unparalleled mastery of botanical drawing even in his early twenties. The renderings of the lily’s six stamens, its six-part envelope of flower petals, and the arrangement of leaves on its stem are completely accurate. “The Madonna lily,” writes Emboden, “is a masterpiece of botanical imagery in every detail.”¹⁸

FIG. 3-4. Study of a tree, c. 1508. Windsor Collection, Landscapes, Plants, and Water Studies, folio 8v (detail).

Leonardo must have produced many more studies of flowers and plants than those we know today in order to be able to paint the Virgin of the Rocks, the Leda, and the complex interlace of luxuriant foliage that covered the vault and ceiling of the Sala delle Asse in the Sforza Castle at Milan.¹⁹ Indeed, art historian Jane Roberts estimates that several hundred studies of plants and flowers from his hand must have been lost.²⁰

The high point of Leonardo’s plant studies was reached in the drawings he produced around 1508–10. As Emboden explains, the unique quality of these works is that they depart strongly from the artist’s earlier studies for paintings and take on the characteristics of independent scientific illustrations.²¹ For example, a drawing of an anemone (Anemone nemorosa) and a marsh marigold (Caltha palustris) in the Windsor Collection (fig. 3-6) represents a fine comparative botanical study in which the flowers of the two species are similar but the forms of their leaves are diverse. On the subsequent folio (fig. 3-7), a rush (Scirpus lacustris) is contrasted with a sedge (Cyperus monti). Both are aquatic plants and are somewhat similar in appearance but belong to different families. In the accompanying text, Leonardo notes the differences between the two species, pointing out in particular the angularity of the sedge’s stem.

FIG. 3-5. Madonna lily, c. 1472–75. Windsor Collection, Landscapes, Plants, and Water Studies, folio 2r.

The transition of Leonardo’s botanical drawings from studies for paintings to scientific illustrations was accompanied by a series of texts that represent his first purely scientific inquiries into the nature of botanical forms and processes. To appreciate the significance of this evolution in Leonardo’s thought we first need to have some idea of the history of botany since antiquity, which formed the intellectual context within which he operated.

Botany from Antiquity to the Renaissance

Throughout antiquity and in the centuries that followed, the study of the living world was known as natural history and those who pursued it were known as naturalists.²² The ideas of the ancients about plants and animals were represented in great detail in the encyclopedic works of four masters—Aristotle, Theophrastus, Pliny the Elder, and Dioscorides—all of which were available to the Italian humanist scholars in printed Greek and Latin editions.

FIG. 3-6. Marsh marigold (left) and anemone (right), c. 1506–8. Windsor Collection, Landscapes, Plants, and Water Studies, folio 23.

Aristotle was the classical author most widely available to Renaissance scholars. His numerous works included several treatises on animals, including the Historia animalium (History of Animals). While Aristotle’s observations of plants were less accurate than his observations of animals, his disciple and successor Theophrastus was a keen botanical observer. His treatise De historia plantarum (Of the History of Plants) was a pioneering work that made him famous as the “father of botany.” However, while Theophrastus was a master of botanical categories, his botany remained purely descriptive. He never inquired into any root causes, and his discussions of environmental influences were scarce and faulty. “He was a great figure in his time,” comments Emboden, “but we must not compare him to any Renaissance botanist, [let alone] to Leonardo da Vinci.”²³

In the first century A.D., the Roman naturalist Pliny the Elder wrote a monumental encyclopedia titled Historia naturalis (Natural History), comprising thirty-seven books, which became the favorite scientific encyclopedia in the Middle Ages and the Renaissance. In this massive compendium, Pliny mentions more than one thousand plants, a number not to be equaled in any book until the Renaissance. However, according to Emboden, “there is no evidence of understanding or inquiry” in any of these numerous entries.²⁴

FIG. 3-7. Study contrasting a rush (top) with a sedge (bottom), c. 1510. Windsor Collection, Landscapes, Plants, and Water Studies, folio 24.

In the subsequent centuries, botany was often considered a subdiscipline of medicine, since plants were mainly studied for their use in the healing arts. For centuries, the authoritative text in this field was the De materia medica (Regarding Medical Materials) by the Greek physician Dioscorides, who was a contemporary of Pliny. It contained references to six hundred plant species, arranged in three categories: aromatic, alimentary, and medicinal. The work was soon translated into Arabic and Latin, and some editions were lavishly illustrated. An exquisite example is the edition known as the Juliana Codex, named after the daughter of a Roman emperor, to whom it was given as a gift. An almost perfect facsimile in the National Library in Vienna remains one of the most beautifully illustrated manuscripts in history.

The Materia medica remained the sole authority for physicians until the Renaissance. No drug was considered legitimate that was not found in it. This doctrinaire use greatly impeded original botanical thought and established botany as a discipline almost exclusively in the service of medicine. Until the sixteenth century, plants would not be investigated as entities in themselves, but merely as accessories to healing and the medical arts. The only other writings on plants discussed their culinary uses or their roles as decorative elements in gardens.

One of the few independent botanical scholars of the Middle Ages was the Abbess Hildegard of Bingen, an extraordinary twelfth-century mystic and polymath who wrote theological treatises as well as botanical and medical texts, composed liturgical songs and visionary poems, and created superb illuminated manuscripts. Two of her books deal with plants in the region of Bingen. They contain original descriptions of seventy plant species, which are identified by their German vernacular names and discussed in relation to medicine.²⁵

Another important medieval botanist was the German scholastic philosopher Albertus Magnus (Albert, Graf von Bollstädt) who lived about one hundred years after Hildegard. Albert collected his botanical observations in a volume titled De vegetabilibus (Of Plants),* which contains several original insights and came to be considered the most important work on botany since Theophrastus. “As a recorder of nature, a morphologist, and the originator of the proper mode of thought in science,” writes Emboden, “Albert is unequalled until we encounter Leonardo.”²⁶

The fifteenth century was the age of the Renaissance herbals—botanical books containing descriptions and illustrations of herbs and plants and their medical properties. With the newly invented printing press, numerous copies of standard texts could be produced, and the use of woodcuts and copper plates made it possible for the first time to reproduce illustrations with complete accuracy.²⁷ Soon large numbers of herbals, patterned after the Materia medica, appeared from presses all over Europe and became extremely popular. Most fifteenth-century herbals went into multiple editions, often under several titles. A single work might be known under many names, which has caused considerable confusion among historians of botany and medicine.²⁸

The scholarship involved in the production of most of these herbals was quite dismal.²⁹ Their main purpose was to show local examples of medical drugs referred to in the classical texts, and misidentifications were very common. The compilers were not concerned about the fact that species of plants found, say, in the Mediterranean were not those of northern Europe. If a plant could not be identified because it did not grow in their region, they would often have a woodcut made of some plant that looked remotely similar and ascribe to it the medicinal properties mentioned in the manuscript from which their book was derived. The vast majority of herbals in the early Renaissance were endlessly repetitive, drawing from one another and from antiquity, but not from nature.

Leonardo the Botanist

At the beginning of the sixteenth century, when Leonardo began his advanced botanical studies, botany was still in a purely descriptive phase and was considered merely an accessory to the healing arts. Even at the great universities of Pisa and Padua, whose professors included some of the leading botanists of the time, no genuine science of botany was taught in which plants were studied for their own sake.

As in so many other fields, Leonardo took his scientific work in botany far beyond that of his contemporaries. Like his fellow humanists, he was very familiar with the texts of the classical naturalists, but he refused to repeat their teachings uncritically.³⁰ Indeed, he despised the established scholars who merely quoted the classics in Latin and Greek. “They strut about puffed up and pompous,” he wrote scornfully, “decked out and adorned not with their own labors but with those of others.”³¹ Leonardo always studied the classical texts carefully and then tested them by subjecting them to rigorous comparisons with his own direct observations of nature.

In contrast to his contemporaries, Leonardo not only depicted plants accurately but also sought to understand the forces and processes underlying their forms. In these studies, often based on observations that were astonishing for their time, he pioneered the emergence of botany as a genuine science. Emboden concludes his extensive analysis of Leonardo’s corpus of botanical ideas with the following assessment:

Collectively, the astute observations by this great “disciple of nature” argue compellingly for his position as one of the greatest thinkers in botany, and a man in advance of his time. Botany as a descriptive science was taken by Leonardo into realms of thought characteristic of the late 17th century and even into the sphere of some 20th-century concepts.³²

Leonardo’s botanical notes are scattered throughout the Codices, and in addition there is a major section on botany in Part Six of the Trattato della pittura (Treatise on Painting), the famous anthology compiled after Leonardo’s death by his disciple Francesco Melzi.³³ As Emboden and other historians have noted, less than half of the material in the Trattato can be found among Leonardo’s remaining manuscripts, indicating that substantial portions of his writings on botany have been lost. In fact, Leonardo scholar Carlo Pedretti has concluded from his thorough analysis of the Trattato’s chronology that Melzi must have copied its botanical sections from an entire lost manuscript on botany written by Leonardo.³⁴

Emboden also points out that the presentation and botanical notation on the sheet depicting a rush and a sedge (see fig. 3-7) suggest a leaf from a treatise on plants, and Pedretti has suggested that Leonardo may have referred to such a treatise on another sheet of the Windsor Collection where he mentions a planned “discourse on herbs.”³⁵ The format of such a manuscript may well have been that of the classical handbooks, but its contents would have gone far beyond those of a traditional herbal. “It would appear,” writes Emboden, “that Leonardo had every intention of writing, or actually executed, a treatise that would explain every aspect of plant growth known to him.”³⁶ Such a book would have been far ahead of its time. The first studies of plants for their own qualities were not published until several centuries later.

At the core of Leonardo’s botanical theory we find, as mentioned earlier, the two grand themes that also appear in the other branches of his science—nature’s organic forms and patterns, and the processes of metabolism and growth underlying them. In subsequent centuries, the investigations of these two themes gave rise to two major branches of modern botany: plant morphology and plant physiology. The term “morphology” was coined in the eighteenth century by the German poet and scientist Johann Wolfgang von Goethe, and its subject, the study of biological form, became the primary concern for biologists in the late eighteenth and early nineteenth centuries.³⁷ The development of plant physiology was triggered by the great advances in chemistry in the eighteenth century. A century later, the perfection of the microscope gave rise to a new branch of botany, plant anatomy, dedicated to the study of the structures and parts of plants, including features invisible to the naked eye. Plant anatomy subsequently expanded into all fields of biology, including molecular biology and genetics.

The development of these branches of botany during the past three centuries reflects a tension that has been present in Western science and philosophy from their beginnings.³⁸ It is the tension between the study of matter (or substance, structure, quantity) and the study of form (or pattern, order, quality). The study of matter was championed by Democritus, Galileo, Descartes, and Newton; the study of form by Pythagoras, Aristotle, Kant, and Goethe. Leonardo clearly followed the tradition of Pythagoras and Aristotle in developing his science of living forms, their patterns of organization, and their processes of growth and transformation.

In his morphological studies, as we shall see in more detail, Leonardo observed and recorded various growth and ramification patterns of flowers and trees. In particular, he noted different arrangements of branches and leaves around the stem—a field of study known in modern botany as phyllotaxis. In his plant physiology he was especially interested in the nourishment of plants by sunlight and water, and the transport of the “vital sap” (umore, or “humor”; sugars and hormones, in modern language) through a plant’s tissues. He correctly distinguished between the two types of vascular tissues known today as phloem and xylem, and he made astute observations about the movement of sap when a tree is injured. Leonardo was also the first to recognize that the age of a tree corresponds to the number of rings in the cross section of its trunk, and that the width of the rings correlates with the wetness or dryness of those years.

Not all of Leonardo’s botanical observations were original, but he always articulated them much better than his contemporaries. Indeed, the botanical sections in the Tratatto della pittura amount to genuine studies in theoretical botany. In the words of William Emboden,

The text was transformed … into a true volume of scientific inquiry which is not paralleled in any similar treatises of that time. Whatever borrowings from the works of others appear, and although there are errors in interpretation in several instances, the work remains an extraordinary document in its experiential verification of phenomena.³⁹

Branching Patterns

Most of Leonardo’s notes in Part Six of the Tratatto della pittura are instructions for painters on how to render plants, trees, and landscapes under varying atmospheric conditions. A good third of this section, however, deals with his morphological studies. In particular, he describes and illustrates various patterns of phyllotaxis that are characteristic of plants and trees.

Leonardo correctly identified the three basic types of ramifications: alternate (branches switching from side to side), opposite (two branches growing in opposite directions from the same node), and spiraled (successive branches rotating through equal angles around the stem). In a sketch in the Trattato (fig. 3-8), he illustrated these three types with the branching patterns of an elm (olmo), an elder (sambuco), and a walnut (noce), respectively.

In view of Leonardo’s lifelong fascination with the spiral as an archetypal pattern of life (see p. 22), it is not surprising that he paid special attention to the branching patterns known today as “spiraling phyllotaxis.” He identified several different types of these spiraling arrangements of leaves on the stem, noting that, in each case, an exact number of rotations around the stem is completed after a certain number of branchings. For example, he pointed out that “nature has arranged the leaves of the last branches of many plants in such a way that the sixth leaf is always above the first, and so it follows successively if the rule is not impeded.”⁴⁰

While studying these branching patterns, Leonardo observed different patterns of flowering. “Some of the flowers that grow on the branches of shrubs bloom first at the very top of these branches,” he noted in the Trattato, “and others open the first flower at the very lowest part of the stem.”⁴¹ While being direct and simple, this observation defines a basic principle* that is still used in botany today to establish taxonomic categories.⁴²

FIG. 3-8. Basic ramification types, illustrated with an elm (right), an elder (center), and a walnut (left). Trattato, chapter 890.

Having identified the basic types of branching patterns, Leonardo proceeded to study the processes underlying their formation. To begin with, he correctly observed that branches and fruits always sprout from the lateral buds located just above the attachment points of leaves. “When the shoot and the fruit of the following year spring from the bud,” he noted in Manuscript G, “the eye lies above and in close contact with the insertion of the leaf.”⁴³ Further on in the same Notebook, he added a lovely metaphor about the ways in which he saw the leaf nourishing and protecting the bud in its axil (the angle formed by the stalk of the leaf and the main stem):

Every shoot and every fruit originates above the insertion of its leaf, which serves as its mother, giving it water from the rains and moisture from the dew that falls at night from above, and often it shields them from excessive heat of the rays of the sun.⁴⁴

Leonardo’s inquisitive mind was not content with the description of the morphology of branching in terms of axils and lateral buds. He wanted to know what causes these buds to grow in specific places, generating specific sequences of branching. He answered this question with a remarkable hypothesis. He suggested that branching patterns have to do with the “humor,” or “vital sap,” that nourishes the plant’s tissues:

Between one ramification and the other, if there are no other particular branches, the tree will be of uniform thickness. And this takes place because the whole sum of the sap that feeds the beginning of this branch continues to feed it until it produces the next branch. And this nourishment, or equal cause, produces equal effect.⁴⁵

This assertion, interlinking the morphology of branching patterns and the physiology of nutrient flow, is indeed quite extraordinary. “It is no small suggestion,” comments botanist Emboden, “that there is a ‘humor’ establishing a cause and effect relationship between the sequencing of branching. What we now know to be hormonally conditioned activity is related to the inactivation of new branches in an area that … supports an existing branch of a high level of activity, and it is here suggested by Leonardo, albeit obliquely.”⁴⁶ With this suggestion, Leonardo was far ahead of his time. “The distancing between branches … went unexplained until the 20th century,” writes Emboden, “when the centers of inactivation generated by hormonal activity became known to botanists.”⁴⁷

Leonardo’s prescient intuition of the causal link between the flow of sap and the patterns of phyllotaxis led him to another highly original observation concerning successive levels of branchings in a tree. At each level, he asserted, the total cross-sectional area of the branches must remain constant. In Manuscript M, he illustrated this rule clearly with two simple sketches (fig. 3-9), and he expressed it succinctly in a passage in Manuscript I. “All the branches of trees,” he notes, “at every stage of their height, when put together, are equal to the thickness of their trunk.” And then he adds: “All the ramifications of the waters, at every stage of their length, being of equal movement, are equal to the size of their parent stream.”⁴⁸

What makes Leonardo’s assertion so remarkable is not so much its intuitive plausibility but the reasoning on which it is based. When a branch grows, Leonardo argues, its thickness will depend on the amount of sap it receives from the area below the branching point. In the tree as a whole, there is a constant flow of sap, which rises up through the trunk and then divides between the branches as it flows upward and outward through successive ramifications. Since the total quantity of sap carried by the tree is constant, the quantity carried by each branch will be proportional to its cross-section, and hence the total cross-section at each level will be equal to that of the trunk.

FIG. 3-9. Ramifications of a tree in which the total cross-sectional area of the branches remains constant at each level. Ms. M, folio 78v.

Leonardo’s argument is typical of the kind of systemic thinking that we find again and again in his science. Having established the conceptual link between the morphology of successive ramifications and the physiology of flowing sap, he then compares this flow of sap to the flow of water through the tributary branches of a river. In his extensive studies of flowing water, he had already discovered and clearly articulated the principle of continuity (see p. 45). Now it was quite natural for him to apply it to the flow of sap in a tree and to deduce corresponding rules of proportion. Moreover, Leonardo applied the same reasoning in his anatomical studies to the flow of blood through branching arteries and veins and to the flow of air through the ramifications of the trachea, comparing both to the branching patterns of rivers and trees.⁴⁹

As far as the ramifications of trees are concerned, modern botany has shown that Leonardo’s rule is not completely accurate, because the flow of sugars and hormones is not the only factor determining the thickness of the branches.⁵⁰ Nevertheless, Leonardo’s intuitive understanding of the causal link between phyllotaxis and the flow of sap, long before the development of biochemistry, is truly impressive.

Plant Growth

For Leonardo, describing “all the forms of nature” with great accuracy and depicting them in magnificent drawings and paintings was not enough. He had to go deeper and understand the nature and causal roots of the processes that underlie living forms and continually shape them. Indeed, the exploration of these causal relationships is one of the main characteristics that distinguishes Leonardo’s research from that of other Renaissance scholars and makes it look so modern to us. In his botany, this meant, as mentioned earlier, that he interlinked the disciplines now known as plant morphology and plant physiology.

In his studies of plant growth, Leonardo explored fundamental questions about many basic processes that are studied by plant physiologists today: how do plants acquire the energy and nutrients necessary for their growth? how do they grow in response to environmental stimulation? what are the pathways of nutrient flow through the plant tissues? how do plants regulate their growth? what are the stages of germination from seed to seedling? In modern botany, these questions are answered in the language of biochemistry and of cellular and molecular biology, involving concepts like photosynthesis, tropism, metabolic pathways, and plant hormones. Leonardo, of course, did not have access to these levels of scientific explanation. But his meticulous observations and great intuition for the nature of organic forms led him to many insights that are remarkably close to modern botanical knowledge.

The ancients believed that plants grew by literally ingesting earth to nourish themselves and increase their mass. Leonardo examined the traditional teachings critically, and to do so he tested them with a simple experiment. He unearthed the roots of a small squash plant and brought it to maturity by supplying it only with water. “I made the experiment … of leaving only one small root on a gourd,” he recorded in Manuscript G, “and this I kept nourished with water, and this gourd brought to perfection all the fruits it could produce, which were about 60 of those long gourds.”⁵¹ From this experiment, Leonardo drew the remarkable conclusion that “the sun gives spirit and life to the plants, and the earth nourishes them with moisture.”⁵²

To appreciate the originality of this statement and the way Leonardo arrived at it, we must remember that botanical experiments were unheard of in the early sixteenth century. As Emboden has noted, it was not until the mid-seventeenth century that an experiment similar to Leonardo’s was carried out. In the 1640s, the Belgian physician Jan Baptista van Helmont planted a small willow tree in an earthenware pot to which he added only water. After five years, Helmont recorded that the weight of the tree had increased dramatically but that the earth had lost only a few grams. He concluded from this that all of the additional plant body had been produced from the water alone.⁵³

Today we know that Helmont’s conclusion was incorrect, since most of the mass produced in plant growth comes from the air. The roots take in water and mineral salts from the earth, and the resulting sap rises up to the leaves, where it combines with carbon dioxide (CO₂) from the air to form sugars and other organic compounds. In this marvelous process, known as photosynthesis, solar energy is converted into chemical energy and bound in the organic substances, while oxygen is released into the air. The bulk of the plant body—including the cellulose and other compounds produced through photosynthesis—consists of heavy carbon and oxygen atoms, which plants take directly from the air in the form of CO₂. Thus, while many people today still tend to believe that plants grow out of the soil, in actual fact most of the plant mass comes from the air.

Both Leonardo and Helmont lived long before the advent of chemistry and hence were unable to recognize the complex processes involved in photosynthesis. However, as Emboden points out, Leonardo came closer to our modern understanding “in suggesting that the sun as well as the moisture from the earth were responsible for the mass of the plant body.”⁵⁴ The critical role of sunlight in photosynthesis was discovered by the Dutch plant physiologist Jan Ingenhouz toward the end of the eighteenth century, and a full understanding of its complex biochemistry was not reached until the twentieth century.

Manuscript G contains another remarkably prescient passage in which Leonardo seems to intuit the role of the atmosphere in the process of photosynthesis. A few pages after the description of his botanical experiment, he notes:

The lower branches, after they have formed the angle of their separation from the parent stem, always bend downward so as not to crowd against the other branches which follow above them on the same stem and to be better able to take in the air which nourishes them.⁵⁵

This passage is noteworthy not only because of the brilliant (and correct) suggestion that plants receive nourishment from the air, but also because it is an example of Leonardo’s observation of tropism, the tendency of plants to orient themselves in response to environmental stimuli. In addition to noting the bending of branches in response to gravity, known to botanists today as geotropism, Leonardo observed the phenomenon of phototropism, that is, the orientation of plants toward light. “The extremities of the branches of plants,” he noted in the Trattato della pittura, “unless they are overcome by the weight of fruit, turn toward the sky as much as possible.”⁵⁶ Both phototropism and geotropism were rediscovered and studied in detail by Charles Darwin at the end of the nineteenth century.

To understand how plants orient themselves and grow in certain ways, Leonardo turned to the flow of sap through the plant tissues, as he had done for the explanation of branching patterns. He used the term “vital sap” for the essential life fluid of plants, and he believed that it nourishes the plant tissues and also regulates their growth. Today we know that the sap contains sugars and hormones and that the latter indeed affect various aspects of plant growth. As Emboden points out, these effects of hormonal activity on the growth of plants were not understood until the twentieth century.⁵⁷ That Leonardo described several of them qualitatively in the early sixteenth century is truly exceptional.

In his studies of trees, Leonardo correctly distinguished between the dead outer layer of the tree’s bark, also known as cork, and the living inner bark, known to botanists as phloem, which he called very aptly “the shirt that lies between the bark and the wood.”⁵⁸ He recognized that the function of this vascular tissue is to transport sap throughout the plant and that it is therefore of critical importance for keeping the plant alive. “In the bark and shirt is the life of the plant,” he noted in the Trattato.⁵⁹ However, Leonardo did not recognize that the transport of water and minerals takes place through the xylem (or wood) inside the phloem, although he identified the inner bark and the wood (the phloem and the xylem) as two distinct tissues. The transport system of the xylem was not known until the late seventeenth century.⁶⁰

A fine example of Leonardo’s precise botanical observations is that of the so-called secondary growth (the increase of a tree’s diameter), in which new cells are created in the phloem and some of them differentiate into cork, which becomes part of the bark while the bark’s outermost layers split apart to accommodate the expansion. Leonardo’s description of this rather complex process is completely accurate:

The growth in thickness of trees is brought about by the sap which, in the month of April, is produced between the inner bark and the wood of the tree. At that time, the inner bark is converted into outer bark, and the outer bark acquires new cracks in the depths of the existing cracks.⁶¹

In his observations of secondary growth, Leonardo also noticed that some of the newly produced cells differentiate into wood, first turning into soft sapwood and eventually into the heartwood that provides the strength of the trunk. He discovered not only that this process generates the annual growth rings in the cross sections of a tree’s branches and trunks, and that the approximate age of a cut tree can be determined by counting those rings, but also—remarkably—that the width of a growth ring is an indication of the climate during the corresponding year. “The rings on the cut branches of trees show the number of their years,” Leonardo recorded in the Trattato della pittura, “and the greater or smaller width of these rings show which years were wetter and which drier.” Then he added, almost as an afterthought: “Although this is of no importance in painting I want nevertheless to describe it in order to leave out as little as possible of what I know about trees.”⁶²

Dove si grida, non è vera scientia. (Trattato della pittura, chapter 33)

Where there is shouting,
there is no true science.

Leonardo was keenly interested in how the flow of sap through a tree’s inner bark affects its growth in various ways. As Emboden has pointed out, these studies are fascinating because several of Leonardo’s observations refer implicitly to hormonal activity, centuries before the discovery of hormones.⁶³ To observe the flow of sap in the phloem, Leonardo paid special attention to injuries sustained by trees. He noted that in such cases, sap rapidly flows to the defense of the injured part, where it stimulates vigorous growth. The grafting of branches offered him ample opportunities to observe this effect. “If a branch of a tree is cut off and there be grafted or inserted one of its own twigs,” he noted, “in time this twig will grow much larger than the branch which nourishes it, because the nourishment or vital saps rush to the defense of the injured place.”⁶⁴ This phenomenon, well known to botanists, is associated today with the collective activity of several types of hormones.⁶⁵

In a passage in the Codex Atlanticus, Leonardo describes the same effect in the case of a tree that has lost part of its bark:

When a tree has had part of its bark stripped off, nature … diverts to the stripped portion a greater quantity of nutritive moisture than to any other part; so that … the bark there grows much more thickly than in any other place. And this moisture has such power of movement that, having reached the spot where its help is needed, it makes various buddings and sproutings, not unlike water when it boils.⁶⁶

As before, Leonardo’s observation of the powerful movement of “nutritive moisture” to the injured area refers implicitly to the migration of sugars and hormones, which produce more tissue in the process of regeneration than in the rest of the bark. The rapidity of the flow of sap serves to seal the area before bacteria and fungi can cause it to rot. As Emboden explains, Leonardo’s “buddings and sproutings after the manner of water when it boils” have been confirmed by modern botanists and are understood today as resulting from high hydrostatic pressures in the special elongated cells of the phloem known as sieve tubes.⁶⁷

In view of his accurate location of the flow of sap in the phloem, or “inner bark,” it is not surprising that Leonardo was well aware of the lethal effect of girdling, in which a tree dies when an entire ring of bark is removed. “If you take away a ring of bark from the tree,” he recorded in Manuscript B, “it will wither from the ring upward, and all below will remain alive.”⁶⁸ As Emboden points out, this statement shows that Leonardo understood that sap is stored in the roots and lower portions of a tree, and that girdling, by preventing any stored sap from flowing upward through the phloem, cuts off the tree’s vital nourishment.⁶⁹ Indeed, the storage of sap in the roots is mentioned explicitly in a note in Manuscript G: “The trunks of the trees have a bulging surface that is caused by their roots, which carry nourishment to the tree.”⁷⁰

In the view of Emboden, Leonardo’s most remarkable observation regarding the effects of the flow of sap on plant growth is expressed in the following note from the Trattato della pittura:

The sap of the branch, when it does not absorb the heat of the sun, falls to the lower part of its branch; and the sap nourishes more where it is in greater abundance.⁷¹

According to Emboden, a modern botanist would interpret this passage as describing the migration of growth hormones, known as auxins, along a branch in such a way that they accumulate on the lower side of the branch owing to the effect of gravity.

Emboden points out that Leonardo’s statement actually includes two distinct observations, both of which are “most extraordinary in [their] correctness.”⁷² One is that some part of the sap can be inactivated by sunlight, which may indeed occur with auxins. The other observation is that the sap, when it is not struck by the sun, “falls to the lower part of the branch.” This migration of auxins from the light to the dark side of a stem has been recognized in modern botany as the fundamental reason that plants bend toward the sun.

We can only marvel at the fact that, long before the discovery of hormones and the advent of biochemistry, Leonardo was able to use his tremendous powers of observation and his great intuition to arrive at a correct qualitative understanding of branching patterns, secondary growth, annual growth rings, phototropism, and the responses of trees to injuries. Like modern plant physiologists, he explained these phenomena in terms of specific peculiarities in the flow of the life fluid of plants through their vascular tissues.

During the years 1508–12, while Leonardo was engaged in his most intensive studies in theoretical botany, he also began to organize his Notebooks, mapped out several comprehensive treatises, and undertook advanced anatomical studies.⁷³ It is therefore not surprising that he often established conceptual links between his botanical observations and his investigations of the “qualities of forms” in other areas. He compared the branching patterns of trees to those of rivers and of blood vessels. He drew spiraling foliage reminiscent of spiraling water vortices. He likened the flow of sap through a plant’s vascular tissues to the flow of blood through human arteries and veins.

One of his most sophisticated comparisons of organic forms in different living systems is to be found in his studies of plant seeds. A sheet in the Windsor Collection contains the following note:

All seeds have an umbilical cord, which is broken when the seed is ripe. Likewise, they have a matrix and secundina, as herbs and all seeds that are produced in pods demonstrate. But those which are produced in nutshells, like hazelnuts, pistachios and the like, have a long umbilical cord, which shows itself in their infancy.⁷⁴

As Emboden explains, Leonardo observed correctly that in flowering plants the seed develops from a structure within the flower’s ovary, known today as the ovule, which remains attached to the ovary wall by a stalk, known to botanists as the funiculus, until it develops into a seed after fertilization.⁷⁵ The “matrix” and “secundina” in Leonardo’s note refer to the outer layers of the ovule, known today as integuments.

What is most remarkable in Leonardo’s observation is his identification of the funiculus (the stalk that attaches the seed to the ovary wall) with the umbilical cord, which attaches the mammalian embryo to the placenta. Having studied the development of the human fetus in great detail and pictured it in superb drawings (see pp. 312ff.), he could not help but be impressed by the delicate structural similarity in the developments of plant seeds and mammalian embryos. It was for him a compelling testimony to the unity of life at all scales of nature. Modern botanists completely agree. They call the tissue within the ovary to which the ovule is attached the placenta and, like Leonardo, they view the funiculus as the plant equivalent of the umbilical cord.

Leonardo’s highly sophisticated observations of intricate botanical forms and his ability to understand them in terms of the underlying processes of metabolism and development puts him far above the natural philosophers of his time. In recognition of this fact, physiologist and Leonardo scholar Filippo Bottazzi concluded his classic essay “Leonardo as Physiologist” with the following homage:

In art he was supreme among the great; in the mechanical sciences, he was the first and foremost restorer. But the story of modern biology begins with him.⁷⁶