THE RELATION OF LEAF AND STEM. A NOTE IN STRUCTURAL BOTANY. BY R. HANSFORD WORTH. (Read at Great Torrington, August, 1899.) THIS paper is intended only as a preliminary note of results attending an inquiry which the author hopes some day to complete in detail. As, however, the facts so far ascertained are of interest, and up to the present the further investigations fully support them, it has been thought well to offer a brief statement on the subject. The method adopted was one of direct measurement and calculation, the explanation of ascertained facts being sought afterwards. Here, however, explanations and theoretical deductions will be set first, and practical confirmation supplied at the last. Leaf and stem are alike integral parts of the plant; of these the leaf is the less permanent institution. Flowering stems may or may not carry leaves in addition to the flowers, and hence may in some cases be entirely dependent on the other parts of the plant. An ordinary stem with its foliage depends on the roots for the great part of the water and the whole of the mineral substances required for its nutrition; but the roots and stem alike are indebted to the leaf for their supply of carbon and carbohydrates. The leaf itself, from the time when it first breaks bud, develops chlorophyll and becomes self-supporting so far as carbon is concerned. Green stems may to some extent be self-supporting also. In the majority of plants, however, the leaves are the great assimilating agents which collect carbon from the air, and manufacture the crude sap, supplied to them by the roots through the intermediary of the stem, into true nutrient sap. The Fungi and other plants devoid of chlorophyll are not now under consideration. Since stem, trunk, and root are alike indebted to the leaves for their nutriment, it is evident that the leaves must manufacture more nutrient sap than is required for their own purposes. The first call upon a leaf is to supply material for its own growth and maintenance, the second to supply material for the stem on which it grows. Since considerable subaerial portions of almost every plant are without leaves, it is obvious that the stem immediately adjoining a leaf must hand on a portion of the nourishment it receives to the branch and trunk below it to maintain and increase their growth, and the trunk again of necessity has to yield up to the roots sufficient for their requirements. A flowering stem devoid of foliage leaves also derives its nourishment from the leaves on other stems. It is reasonable to suppose that during its period of growth each leaf retains for its own use a considerable percentage of the nutrient materials it manufactures; after attaining its full growth it retains little or none. The stem, even during the growth of the leaf, is itself growing, as is necessary firstly to provide a support sufficiently strong for the constantly increasing leaf-area, secondly to carry the leaf as it increases in area further away from the adjacent leaves, and thus avoid overlap and overcrowding. This growth of stem is maintained in strict unison with the requirements of the leaf by the fact that the leaf itself, as the provider of nourishment, regulates by its size the amount of nutrient material provided to the stem, and the growth of the stem is proportionate to this amount. So far we are dealing with a terminal leaf and the section of stem lying between it and the next. By the growth of the terminal leaf and its stem interleaf a further burden is put on the remaining stem, which must be strengthened to endure it; this is provided by the surplus from the first interleaf, which only absorbs a percentage for its own growth. The trunk bearing the stem again requires to be strengthened to bear the increased weight of the stem as a whole, and the nourishment for this is derived from the stem, which takes toll only on the nourishment sent on to it by the first interleaf. The root system has to be expanded to meet the growing wants of the plant, and the material for this expansion is supplied by the trunk, which retains only as much as it needs of the nourishment sent on to it by the stem, and so the surplus nourishment from the leaf is finally utilised, or, if not immediately required, may be stored for use. So far, we have spoken of strength only, but a further element has to be considered-the provision for passage of water and substances in solution to and fro between the roots and leaves. All other things being equal, this carrying capacity of a stem depends on its cross-sectional area, which in cylindrical stems varies as the square of the diameter. The more leaves, therefore, the greater should be the diameter of the stem. Subject to further qualification, the square of the diameter of the stem at any point should always have a constant ratio in each plant to the sum of the areas of the leaves beyond that point. In a plant of simple growth the leaves occur at intervals along the stem, which intervals bear a definite ratio to the width or length of the leaves. The stem between the points of attachment of any two leaves is for present purposes called an interleaf. Examine the stem of a hazel or other convenient plant, and it will be found that each interleaf is of uniform diameter throughout its length. The stem does not taper in the form of an elongated cone, but is made up of a series of cylinders of diameters constantly increasing towards the trunk; each increase in diameter takes place at the point of attachment of a leaf or shoot. This is precisely equivalent to the case of a water-main with a series of small supplies led into it at intervals; where each supply joined the main an increase in its diameter would be necessary to enable it to convey the enhanced quantity. It is not in strict accord with the requirements of strength, which demand a longitudinal geometrical taper throughout the stem, and not sudden accessions at intervals. strength of the stem at any point is proportionate to the fourth power of its diameter, and should bear a constant ratio to the sum of the moments of all the leaves up to the end of the stem about that point, plus some allowance for the weight of and wind pressure on the stem itself. It is obvious that, should the stem either prove insufficient to carry the fluids required, or unable to support itself and the leaves, the plant cannot continue to live unless in the latter case it is either of a trailing or climbing habit. The The strength of the stem does not, however, vary directly as the fourth power of the diameter throughout its length. At and near the growing point the woody fibre is not fully developed, and hence there is no fair comparison between the younger and the older portions. The final "how" is rarely ascertainable in Nature's |