144 THE PLANT 
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 respec- 
tively.t 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 chlor- 
enchym 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 re- 
sulting in a central sponge tissue, or water- 
storage tissue. The spongophyll, or sponge 
leaf, is regularly a shade type; the chlor- 
enchym 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 pali- 
Fig. 44. A stable species, sade tissue. Diphotic leaves always contain 
Erigeron speciosus: 1, sun leaf; 2, 4 5 
shade leaf (light .03).. X 130. both palisade and sponge, though the ratio 
between .them varies considerably. Diphot- 
ophylls are characteristic of sunny mesophytic habitats. They are 
frequent in xerophytic habi- 
tats as well as in woodlands 
where the light is not too 
diffuse. In the case of stable <3 Xe 
species, this type of structure Gagne 
sometimes persists in the dif- Cy rcoeaae 
fuse light of coniferous forests. 7X .f Va" Fo] 
Floating leaves, in which the C2: Ae: é 
light is almost completely cut { : 
off from the lower surface, are 
also members of this group. 
Submerged leaves, on the other re : 
hand, are spongophylls. wig ie ‘ ay Spongephyll of Gyrostachys stricta 
182. Heliophytes and sciophytes. The great majority of sun plants 
possess diphotophylls. This type is represented by Pedicularis procera 
Sz 
1CLEMENTS, E.S. The Relation of Leaf Structure to Physical Factors. 1905. 
