=181. Types of leaves.= Isophotic leaves are equally illuminated and possess more or less uniform chlorenchym. Diphotic leaves are unequally illuminated, and exhibit a differentiation into palisade and sponge tissues. They may be distinguished as isophotophylls and diphotophylls respectively.[18] Isophotic leaves fall into three types based upon the intensity of the light. The staurophyll, or palisade leaf, is a sun type in which the equal illumination is due to the upright position or to the reflection from a light soil, and in which the chlorenchym consists wholly of rows of palisade cells. The diplophyll is a special form of this type in which the intense light does not penetrate to the middle of the leaf, thus resulting in a central sponge tissue, or water-storage tissue. The spongophyll, or sponge leaf, is regularly a shade type; the chlorenchym consists of sponge cells alone. For the present at least it is also necessary to refer to this group those monocotyledons which grow in the sun but contain no palisade tissue. Diphotic leaves always contain both palisade and sponge, though the ratio between them varies considerably. Diphotophylls are characteristic of sunny mesophytic habitats. They are frequent in xerophytic habitats as well as in woodlands where the light is not too diffuse. In the case of stable species, this type of structure sometimes persists in the diffuse light of coniferous forests. Floating leaves, in which the light is almost completely cut off from the lower surface, are also members of this group. Submerged leaves, on the other hand, are spongophylls. =182. Heliophytes and sciophytes.= The great majority of sun plants possess diphotophylls. This type is represented by _Pedicularis procera_ (fig. 32). Plants with isophotophylls are found chiefly in xerophytic places, though erect leaves of this type occur in most sunny habitats. The staurophyll, in which the protection is due to the extreme development of palisade tissue, is illustrated by _Allionia linearis_ (fig. 40) and _Bahia dissecta_ (fig. 33). The diplophyll, which is characterized by a central band of sponge tissue or storage cells, is found in _Mertensia linearis_ (fig. 34). The form of the spongophyll that is found in certain monocotyledons is shown by _Gyrostachys stricta_ (fig. 45). The spongophyll (fig. 38:3, 39:2) is frequent among plants of deep shade, but as the leaf sections of _Allionia_ (figs. 38, 40) and _Quercus_ (fig. 42) show, the diphotophyll is the rule in shade ecads. EXPERIMENTAL EVOLUTION