ECOLOGY OF •IIE LEAF. Ecologically, the leaves of plants may be considered from the standpoint of the various leaf-forms found in nature, and (2) from that of the evidence ob tained by experiments to determine the cause of the various forms. In general, leaves are expanded organs. and they also have a posi tion which is in most cases perpendicular to the majority of incident rays of light. This position is technically termed diaheliotropie. It can readily be seen that these conditions favor the absorption of the largest quantities of radiant energy; and, inasmuch as radiant energy is essential to the growth and life of plants, it is clear that the expanded form and the diaheliotropie position are distinct ly advantageous. though it by no means follows that the need for light has caused either the form or the position. Large numbers of leaves are finely divided; this is conspicuously true in the hydrophytes (q.v.), but it is also true of a vast number of plants with aerial leaves. While we can hardly believe that compound leaves have been caused in any such way, it is nevertheless true that a larger amount of leaf surface can he presented to the sunlight than in the ease of plants with entire leaves. Perha'ps the most advantageous leaf type of all is that which is illustrated by the grasses; here vertical leaves or leaves which are approximately vertical are grouped together in vast numbers, probably se curing the greatest leaf surface in a given space that is found anywhere in the plant kingdom. Simple experimentation shows, however, that the vertical position of grass-leaves, at least in many cases, is due to mechanical ea uses and has little or no relation to light stimuli. Another condi tion which favors the admission of light is the presence of petioles. Maple-leaves which are de veloped in the strong sunlight have short petioles, while more shaded leaves acquire long petioles and thus ultimately get into a well-lighted posi tion, if that be possible. Petioles thus give a much greater plasticity and flexibility to leaves. Other leaves have the power of motility which is strikingly illustrated in the sensitive plant and clover. The advantage of this motility is not al together clear, especially since the closing of the leaf occurs chiefly in the night rather than in the day. (For a further discussion of this topic, see XEROPHYTES.) The placing of the leaves on the stems also varies to a high degree in nature, and in general there seems to he a sort of rela tion between the phyllotaetie arrangement and type of leaf arrangement, since the outer leaves are more commonly farther apart than small ones. Rosette plants present a very interesting type of leaf arrangement, since the outer leaves often have long petioles and the inner ones none at all; not only this. but the phyllotactie arrange ment is such that the shading of one leaf by another is largely prevented. In one way or another, then, it seems that there is a general tendency in nature for plants to dispose their leaves in such a. way as to prevent shading. It is very do9btful, however, if this can be regarded as a direct result of the light stimulus. If in nature leaves are not seen to shade each other to any great extent. it may not be that this is an adaptation by natural selection, but rather that the leaves which were shaded have been compelled to die through getting insufficient food; this results in a survival of the unshaded leaves. A
study of the leaves of a patch of rank weeds shows that all the lower leaves have died and that only the uppermost have been able to en dure.
Recent experimental studies on leaves by Goebel, Brenner, and others have contributed much to the solution of the question of the causes of leaf shape and form. Goebel's theory is that light is the chief factor in the matter. His experiments on cacti and on the harebell have shown that if the light is weak large leaves de velop. whereas strong light develops small leaves. Other experiments which have been made more recently throw grave doubt upon Goebel's results; in the first place, many instances have been cited in which light favors rather than retards the development of leaves; and secondly, other fac tors which Goebel did not recog,nize have, even in the very forms which he studied produced the very results which he referred to light (Fig. 4). Kohl and more recently Brenner have considered that moisture is the chief element which affects leaf shape. particularly the relation which exists be tween absorption and transpiration. To illus trate. in a moist atmosphere leaves grow large because the transpiration is checked and the turgor is thereby increased. On the contrary, in a dry air leaves grow small because the transpiration is increased and the turgor re duced. In other words, anything which tends to increase or decrease cell turgescence tends to modify not only the leaf size but also the leaf form itself (Fig. 5). It is not possible at present, on account of the very small number of experi ments, to reach any very definite conclusions. A great many differenees which leaves show cannot now he referred to any mechanical cause, but it surely seems to be the present tendency to adopt an explanation of this kind for the variations in leaves rather than to give a teleological explana tion.
The structure of leaves varies as well as the external form, and here also two prominent theories have been advanced to account for the changes observed. The chief changes which have been observed are associated with the chlorophyll cells and the epidermis. Stahl in particular has held that increased light causes the development of palisade cells. This view has been rather generally accepted and at present there seems to be no valid reason for serious objection to it. It may, however, rightly be a subject for further investigation to settle the question of the precise effect of light upon eell-strueture. Leaves which are grown in a moist atmosphere develop a thin wall, whereas a thick wall is developed in a dry atmosphere. Perhaps the cause in the latter ease is to be referred to the greater concentration produced by excessive transpiration. This might lead to a deposition of eell-wall material. Sto mata, as a rule, are less subject to experimental change than are the other leaf-organs. However. some plants, as the mermaid weed. do not de velop stomata when submerged. The stimulating cause for this change has not been suggested. For bibliography, see BOTANY. Sec also Ms