210 



PLANT PHYSIOLOGY 



cell into soluble, portable, and useable compounds (see 

 pp. 29-30). If the work of the enzyms is interfered with 

 by light, the functions of the cell must be altered or become 

 deranged. The substances composing the living protoplasm 

 may also be as sensitive to light as its products. The sensi- 

 tiveness to light of the com- 

 pounds in and perhaps compos- 

 ing their cells is the reason for 

 the sensitiveness of living organ- 

 isms to light. 



The influence of light, like the 

 influence of gravity, may show 

 itself in the kind, rate, and direc- 

 tion of growth, and in the posi- 

 tion of the organ and organism. 

 Plants kept in darkness will be 

 longer than plants under nor- 

 mal illumination. Seeds sprouted 

 in darkness, and seedlings grow- 

 ing where no light falls upon 

 them, grow under the influences 

 of all other forces than light. 

 When the influence of light is 

 wholly eliminated, root, stems, 

 and branches grow longer but 

 are proportionally more slender 

 than in ordinary sunlight, the 

 leaves are smaller and weaker, 

 flowers do not form (see pp. 

 271-4). That stems are more 

 slender and mechanically weak- 

 er in darkness than in light 

 may be due to the less than nor- 

 mal weight of the small weak leaves to the absence of me- 

 chanical strain (see p. 188). The leaves require a certain 

 amount of light in order properly to develop the food- 

 manufacturing tissues. Yet, comparing the total growth of 

 plants of the same species in light and in darkness, it will 

 be clear that the growth is greater in darkness so long as 



Figure 15. Branch of Cactus, 

 the young parts of which grew 

 in darkness. (From Goebel.) 



