PHOTOHARMOSE I39 



the leaf is largely determined by the chloroplasts acting through the cells 

 that contain them. A preponderance of sponge tissue produces an extension 

 of leaf in the direction determined by the arrangement of the plastids and 

 the shape of the sponge cells, viz., at right angles to the light. Shade leaves 

 are in consequence broader and thinner, and sometimes larger, than sun 

 leaves of the same species. A preponderance of palisade likewise results 

 in the extension of the leaf in the line of the plastids and the palisade cells, 

 i. e., in a direction parallel with the incident ray. In accordance, sun leaves 

 are thicker, narrower, and often smaller than shade leaves. 



178. Form of leaves and stems. In outline, shade leaves are more 

 nearly entire than sun leaves. This statement is readily verified by the 

 comparison of sun and shade ecads, though the rule is by no means without 

 exceptions. In the leaf prints shown in figures 14 and 15, the modificati"on 

 of form is well shown in Bursa and Thalicinim; in Capnoidcs the change 

 i? less evident, while in Achilleia and Machaeranthera lobing is more pro- 

 nounced in the shade form, a fact which is, however, readily explained when 

 other factors are taken into account. The leaf prints cited serve as moie 

 satisfactory examples of the increase of size in consequence of an increase 

 in the surface of the shade leaf, although the leaves printed were selected 

 solely with reference to thickness and size or outline. In all comparisons 

 of this kind, however, the relative size and vigor of the two plants must be 

 taken into account. This precaution is likewise necessary in the case of 

 thickness, which should always be considered in connection with amount of 

 surface. The relation between surface and thickness is shown by the follow- 

 ing species, in all of which the size of the leaf is greater in the shade than in 

 the sun. In Capnoides anreum, the thickness of the shade leaf is ^^ (6:12) 

 that of the sun leaf; in Galiiun borcale the ratio is 5:12, and in Allionia 

 linearis it is 3:12. The ratio in Thalicfrum sparsiUorum is 9:12, and in 

 Machaeranthera aspcra 11:12. The thickness of sun and shade leaves of 

 Bursa bnrsa-pastoris is as 14:12, but this anomaly is readily explained by 

 the size of the plants ; the shade forni is ten times larger than the sun 

 form. Certain species, e. g., Erigeron speciostis, Potentilla hipinnatiMa, 

 etc., show no change in thickness and but little modification in size or out- 

 line. They furnish additional evidence of a fundamental principle in 

 adaptation, namely that the amount of structural response is profoundly 

 affected by the stability of the ancestral type. 



The effect of diffuse light in causing stems to elongate, though known for 

 a long time, is still unexplained. The old explanation that the plant stretches 

 up to obtain more light seems to be based upon nothing more than the co- 

 incidence that the light comes from the direction toward which the stem 

 g^ows. Later researches have shown that the stretching of the stem is due 



