198 



Plant Physiology 



and form. It characterizes also to a very high degree the 

 algae and mosses, but it is absent from all the fungi. Sev- 



\\ 



eral facts regarding the habi- 

 tats and distribution of green 

 plants afford us an indication 

 of some of the conditions req- 

 uisite for the proper work 

 of plants thus endowed. The 

 presence of the green color 

 referred to is universally indic- 

 ative of the possession of 

 chlorophyll, a mixed pigment, 

 imbedded in certain chlo- 

 roplasts, or chlorophyll con- 

 taining bodies which are differ- 

 entiated portions of the living 

 protoplasm. In particular, it 

 is apparent that plants con- 

 taining this substance are sun- 

 loving or at least light-loving 

 organisms. They may grow 

 in partial shadow at times, but 

 they are wholly absent from 

 all permanently dark or deeply 

 shaded places. The large sur- 

 faces of the leaves and the evident arrangement of these 

 and of the branches which bear them, with respect to light, 

 all indicate clearly a certain relation of green color with the 

 light factor. As the chief bearers of chlorophyll in seed- 

 plants the leaves command special attention, wholly aside 

 from their other functions or accessory work. Any agency 



FIG. 52. Cell of chlorenchyma 

 showing chloroplasts with starch 

 grains. [Adapted.] 



