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 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 pronounced 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 following 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 ½ (6 : 12) that of the sun leaf; in _Galium boreale_ the ratio is 5 : 12, and in _Allionia 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 outline. 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.