PHOTOHARMOSE 139 
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., ina 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 modification 
of form is well shown in Bursa and Thalictrum; in Capnoides the change 
is 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 more 
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 aureum, the thickness of the shade leaf is % (6:12) 
that of the sun leaf; in’ Galium boreale the ratio is 5:12, and in Alhoma 
linearis it is 3:12. The ratio in Thalictrum sparsiflorum is 9:12, and in 
Machaeranthera aspera 11:12. The thickness of sun and shade leaves of 
Bursa bursa-pastoris is as 14:12, but this anomaly is readily explained by 
the size of the plants; the shade form is ten times larger than the sun 
form. Certain species, e. g., Erigeron speciosus, Potentilla bipinnatifida, 
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 
grows. Later researches have shown that the stretching of the stem is due 
