184 



PLANT PHYSIOLOGY AND ECOLOGY 



of sun plants are diphotic, as represented by Pedicularis procera 

 (Fig. 54). Plants with isophotic leaves are found frequently 

 in xerophytic places, though erect leaves of this type occur in 



Fig. 65. Isophotic leaf (1) of Bidens bigelovii, which in the shade form (2) 

 becomes practically a spongophyll by the reduction of the palisade 

 layer. 



most sunny habitats. The staurophyll, in which protection is 

 due to the extreme development of palisade tissue, is illustrated 

 by Allionia linearis (Fig. 58) and Bahia dissecta (Fig. 47). The 

 diplophyll, which is characterized by a central band of sponge 

 tissue or storage cells, is found in Mertensia linearis (Fig. 50). 

 The spongophyll is frequent among plants of deep shade, but, as 

 the leaf sections of Allionia (Fig. 58) and Quercus (Fig. 64) show, 

 the diphotic leaf is equally common among shade plants. The 

 form of sun spongophyll found in certain monocotyledons is 

 shown in Gyrostachys striata (Fig. 59). 



Experiment 57. Sun and shade forms in nature. Make a list of all 

 the genera of the local flora in which sun and shade species occur, and 

 make a general comparison of the latter. . ., 



Make a thorough search for sun and shade forms of the same species. 

 Such species, called polydemics, are especially apt to be found near 

 the border of grassland and woodland, where species may wander easily 

 into either habitat. Make a study of the sun and shade forms of a 

 polydemic by means .of outlines and leaf prints. 



