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,^ 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 M^holly 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- 

 sade tissue. Diphotic leaves always contain 

 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 

 species, this type of structure 

 sometimes persists in the dif- 

 fuse 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 ^ ^ 



, , 1 Fig. 45. Spongophyll of Gyrostachys strida 



hand, are spongophylls. (light l). X 130. 



182. Heliophytes and sciophytes. The great majority of sun plants 

 possess diphotophylls. This type is represented by Pedicnlaris proccra 



Fig. 44. A stable species, 

 Erigeron speciostis: 1, sun leaf; 2, 

 shade leaf (light .03). X 130. 



'Clements, E. S. The Relation of Leaf Structure to Physical Factors. 1905, 



