Leonardo da Vinci - the anatomist (1930) 21
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McMurrich JP. Leonardo da Vinci - the anatomist. (1930) Carnegie institution of Washington, Williams & Wilkins Company, Baltimore.
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Leonardo da Vinci - The Anatomist
Chapter XXI Botany
In considering Leonardo as an Anatomist one thinks of him as an investigator of the structure of the human body, accepting what has become the popular significance of the word anatomy. But, as has been seen, in his search for light Leonardo extended his studies to other animals and, further, he included in the scope of his observations the other great group of living organisms, the plants. Like animals, they too had vital spirits, contributed in their case by the sun, and they too required nourishment, derived in their case from the earth’s humidity (Cf, 32v; TP, 823). He found, too, other interesting analogies between plants and animals. He compares, for instance, the coverings of the brain, scalp, cranium and membranes, to the coats enclosing the germ of an onion (QV, 6) ; he likens the arrangement of the blood-vessels to a tree (QV, 1) and finds an analogy between the heart with the great vein and a germinating nut with its ascending plumule and descending radicle (AnB, 1 1). It seems fitting accordingly that his botanical observations should find some brief consideration here.
Numerous beautiful drawings of trees and flowering plants indigenous to Italy are to be found in his manuscripts. These, however, are not of present concern; his observations on plant structure and plant physiology are the matters of interest. The chemistry of the day could give him little insight into the processes of nutrition in plants and he seems to have regarded the rain, the dew and the moisture contained in the soil as the source of the plant’s food.
“Plants are nourished in summer through their leaves by the dew and the rain, and in winter by the contact they have with the soil by means of their roots.” (TP, 856.)
But it was the moisture of the soil and not its mineral constituents that was of importance, for he states explicitly that it is the moisture of the soil that constitutes their nourishment (G, 32 v). Further he describes an experiment in which he left a gourd ( zucha ) with only a very small root and it was nourished only with water and yet it brought to perfection all its fruits, about sixty.
“And I gave diligent attention to this great vitality and recognized that it was the nightly dew that with its moisture penetrated at the attachment of its large leaves to nourish the plant and its progeny.” (G, 32v; TP, 823.)
The concluding sentence of this quotation implies that Leonardo had observed the relation of the buds to the leaves, namely, that they occur in the axils of the leaves. But the observation is recorded more definitely in the statement that each branch and fruit arises above a leaf, which serves as a mother in bringing the rain and the dew and in protecting from the sun (G, 33v). And he speaks of the leaf, not only as the mother of the bud, but also as its teat or udder (G, 32v; TP, 823). The significance of the relation is more fully expressed as follows:
“Because the branch or fruit arising in the following year from the bud veinlet of the eye which is above the contact of the attachment of the leaf, the moisture that bathes the branch can descend to nourish the bud by the drop being held in the concavity of the origin of the leaf.” (TP, 822.)
With this idea as to the meaning of the axillary position of the buds it was not a great step to inquire how far the arrangement of the leaves favored the access of moisture to the buds and thus Leonardo took the first step in the determination of the laws governing the arrangement of leaves, the law r s of phyllotaxis, later further worked out by Grew and Malpighi in the eighteenth century and by Sachs in the nineteenth. He noted that in many plants the sixth leaf was above the first (G, 16v; TP, 828), that is to say the leaves were arranged in a spiral along the branch or stem, so that in passing from any leaf to the next directly above it, through the points of attachment of intervening leaves, two turns would be made around the stem, and the angle of divergence of one leaf from the next succeeding one would be two-fifths of the circumference of the stem. This he regarded as the most frequent arrangement and mentioned the vine, canna and pruno de more as examples of it; in the white jasmine ( Vitalbo gelsomino ), however, the leaves are arranged opposite one another in pairs, successive pairs being at right angles (G, 16v; TP, 828). In another passage it is stated that there are four modes of arrangement, but only three are mentioned: (1) that in which the sixth leaf is above the first, (2) that in w T hich the two-thirds above are over the two-thirds below, and (3) that in which the third above is over the third; below (G, 33 ; TP, 828) . The first, of course, is the two-fifths arrangement; the third is probably the onethird although it might be interpreted as the one-half; but what is meant by the second arrangement is by no means clear, perhaps it was the three-eighths. If it were, then Leonardo had discovered the principal varieties of phyllotaxis, and in the manner of his time he finds a teleological explanation for the arrangements in that they allow the free access of air, light and moisture to the leaves; the alternation—
“Serves (1) to allow intervals for air to pass between; (2) that drops falling from the first leaves may fall on the fourth and sixth of other branches.” (TP, 905.)
“(1) That the branch or fruit coming from the bud that is above in contact with the attachment of the leaf may be nourished by the water that moistens the branch, (2) that the leaves do not overshadow one another.” (G, 16v.) »
Since the branches arise from buds in the axils of the leaves, they too will show a phyllotactic arrangement in an untrimmed tree, provided, of course, that every bud develops, and it may therefore be possible to determine the age of a tree by counting the series of branches (TP, 820). It is stated that branches are arranged in two ways, either above one another or not (TP, 819), and in another passage (TP, 813) eleven principles affecting the arrangement of branches are laid down; (1) Every branch of a tree, if not prevented by its weight, curves its tip toward the sky; (2) the lower branches are larger than the upper; (3) all the branches at the center of the tree are soon killed by being in the shade ; (4) the branches nearest the top of the tree will be more vigorous on account of their access to the air and sun; (5) the angles at which the branches form are all equal; (6) but they become more obtuse as the branches thicken in growing old; (7) the width of the angle is more oblique in slender branches; (8) the branches of a bifurcation are together of equal size with the branch from which they arise ; (9) the twistedness ( tortura ) of the large branches is proportional to the development of their branches that do not interfere with one another; (10) the twistedness of that branch is greatest which has its branches most equal in size; (11) the attachment of a leaf always leaves a mark below its branch, growing with the branch until the bark cracks and fissures from the age of the tree. What Leonardo meant by items 9 and 10 is not clear; 2 and 3 he expands somewhat in other passages, which are interesting as examples of Leonardo’s search for natural explanations of natural phenomena.
“Always the largest branches are those that grow from the part that looks toward the ground and the lesser those that grow from the upper part. Because the moisture ( omore ) of the branches, when it is not percussed by the heat of the sun, flows down to the lower parts of the branches and the moisture nourishes more where it is in greater abundance. And such a branch always has its bark thicker below than above.” (T P, 832.)
“The branchings of the larger branches do not occur toward the middle of the plant. And this is so because naturally every branch seeks the air and avoids the shade and the shade is greater in the lower parts of the branches that look toward the earth than in those that are turned to the sky. In these the water that rains and the dew that abounds at night flows down and keeps the lower parts more moist than the upper, so that the branches have more nourishment in those parts and thus grow more.” (TP, 831.)
The growth in diameter of an exogenous stem or branch was ascribed to —
“the sap that forms between the bast ( camicia ) and the wood, in the month of April, at which time the bast is converted into bark, which acquires new fissures at the bottom of those already there.” (TP, 833.)
The number of branches that may be produced from a branch depends on the quantity of bast between the bark and the -wood (TP, 829), “the thing nourished is as its nourishment” (TP, 830). If a branch be cut off and another branch from the same tree be inserted in its place, the graft will in time become somewhat larger than the branch that nourishes it, since the nourishment and vital spirits succor the injured place. If many eyes of plants be inserted in a circle as grafts on a cut trunk they will acquire in the same year a greater size than had the trunk that w y as cut away (TP, 830). Other things being equal, however, the nourishment would be equally distributed, so that when branches are opposite one another they will be of equal size (TP, 837), and, furthermore, the sum of the sizes of the branches of any year will be equal to the size of the branch grown the year before and so on in the future (TP, 817).
Not less striking than the discovery of phyllotaxis was the observation that the age of a tree or branch might be determined by counting the growth rings on a cross-section. And not only so, but by observing the relative widths of the rings, whether they are broad or narrow, the nature of the season, whether it was wet or dry, might be ascertained. Furthermore, the surface of the tree that faced the south might be distinguished by the greater breadth of the rings on that portion of their circumference, since in the northern hemisphere the southern surface is that most exposed to the sun and therefore better supplied with nourishment. This difference in the widths of the individual rings brings it about that the axis of the tree lies a little nearer the southern than the northern surface (TP, 820).
The roughness of the bark is also explained by the exogenous mode of growth. In young shoots the bark is smooth, but in older branches it becomes corrugated and is roughest on the oldest branches (TP, 820), and rougher on the southern surface, where growth is most active than on the northern (TP, 835). The roughness is due to the splitting or cracking of the bark as the stem or branch increases in diameter, and the splitting is always lengthwise, except in the cherry ( ciliego ) in which it occurs in circles (TP, 819).
The favoring influence of the sun on growth has been already noted and is definitely indicated in the statement that plants which see the sun set seed, those that only see its reflexion do not (G, 37 v). Its influence resulted in increased growth activity, such as that witnessed by the supposed greater width of the growth rings and the greater fissuration of the bark on the southern side of a tree. And it was such phenomena that led to the statement that plants always curve with the convexity toward the south (G, 36v), a statement that has gained for Leonardo the credit of having observed the phenomena of heliotropism in plants. As a matter of fact heliotropism bends the plant toward the source of light, not away from it; the effect of exposure to the sun is rather to inhibit than to increase growth. Leonardo regarded the sun as the source of the vital spirits of plants, and if this were so it followed that those parts of the plant most fully exposed to the sun would have the greater vitality and therefore greater growth. That this was his train of thought is indicated by the continuation of the passage just mentioned (G, 36v), as it states that the branches on the south side are stronger and longer than those on the north side, and he explains both this and the supposed curvature on the ground that the sun attracts the humor toward that surface of the plant which is nearest to it.
But, nevertheless, Leonardo did record the occurrence of heliotropism when he stated that the branch of a tree, if not prevented by weight, curves its tip toward the sky (TP, 813), a statement in direct opposition to that on G, 36v. Furthermore he records what may be regarded as an example of negative heliotropism when he notes that old trees cover the bark of the north side of their trunks with a greenish featheriness ( verdicante piumosita ) (TP, 834), meaning thereby, of course, the moss which, in the northern hemisphere, grows most abundantly on the north side of trees.
In his studies of plants, then, Leonardo showed the same keenness of observation as has been seen in his animal studies, and in these too he was sometimes led astray by faulty physiological theories. Further he applied the experimental method in his plant studies as has been seen in the experiment with the gourd mentioned on p. 244, but a more pretentious one is described in the Codex Atlanticus. He bored a small hole in the trunk of a tree and poured in arsenic and sublimate dissolved in spirits of wine, expecting either to make the fruit poisonous or to destroy it. When the fruit was ripening the hole was deepened until the pith was reached and the poison forced in by a syringe. Unfortunately the results of the experiment are not recorded.
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Reference: McMurrich JP. Leonardo da Vinci - the anatomist. (1930) Carnegie institution of Washington, Williams & Wilkins Company, Baltimore.
Cite this page: Hill, M.A. (2020, July 14) Embryology Leonardo da Vinci - the anatomist (1930) 21. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Leonardo_da_Vinci_-_the_anatomist_(1930)_21
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