34 Plant Life and Evolution 



are familiar to every one. The bleached potato 

 sprouts in a cellar, or the yellow blades of grass 

 under a board, are sufficiently striking examples of 

 the effect of the exclusion of light. As might be 

 expected, the parts most conspicuously affected are 

 those directly associated with photosynthesis. This 

 is seen in the great reduction of the leaf surface and 

 the absence of chlorophyll in most of the higher 

 plants when they are grown in darkness. 



Reaction of Unicellular Plants to Light. The 

 modifications of the plant body associated with light 

 adaptation are by no means confined to the higher 

 plants. Unicellular plants often react very promptly 

 to light, moving toward the light when they are 

 motile or shifting the position of their chromato- 

 phores according to the direction and intensity of 

 the light rays. A similar shifting of the chromato- 

 phores in response to light may be demonstrated 

 also in the cells of the higher plants. The form of 

 the chromatophores also may be explained as a case 

 of light adaptation. They are most commonly flat- 

 tened discs or thin plates of various forms, thus 

 exposing large surfaces to the light rays. 



Photosynthetic Organs of the Lower Plants 

 Low down in the scale of plant life there is evidence 

 of the development of special structures associated 

 with photosynthesis. Two types of photosynthetic 

 adaptation are met with among the lower algae. In 

 some of these, like the sea-lettuce (Ulva), the whole 

 plant has the form of a thin lamina or thallus, thus 



