PLANT PE7SI0L0OY. 83 



or absence of sunlight. In the sunlight the absorption of 

 carbon dioxide in carbon-assimilation is so greatly in excess 

 of its exhalation as a result of metabolism, that the latter 

 is unnoticed. In darkness, however, when carbon-assimi- 

 lation is stopped, the exhalation of carbon dioxide becomes 

 quite evident. 



151. So, too, with oxygen: in the sunlight. its evolution 

 from carbon-assimilation is so greatly in excess of its ab- 

 sorption (for metabolism) that the latter was long unknown; 

 but in the absence of light its absorption becomes manifest. 

 Parasites and saprophytes, as well as those parts of ordinary 

 plants which are wanting in chlorophyll, as flowers and 

 many fruits, deport themselves in this regard exactly as 

 chlorophyll-bearing organs do in darkness. 



152. Division of Labor. — In homogeneous-celled holo- 

 phytes (i.e., green plants whose cells are all alike), whether 

 few- or many-celled, every cell performs all the operations 

 noted above; but in heterogeneous-celled holophytes there 

 is a division of labor, some cells or masses of cells engaging 

 in certain activities quite difEerent from those engaged in 

 by other cells or tissues. 



153. Nutrition of Moss-like Plants. — In a moss the cells 

 of the root-hairs (rhizoids) which clothe the subterranean 

 part of the stem engage in the absorption of watery solu- 

 tions almost exclusively, and since they do not take part 

 in carbon assimilation they are destitute of chlorophyll. 

 On the other hand, the cells in the leaves are active in 

 carbon assimilation, and have an abundance of chlorophyll. 

 They absorb carbon dioxide from the air and but very little, 

 if any, water or soluble food-matter. The cells of the 

 leaves and stem must therefore obtain their supply of watery 

 solutions from the cells,, in the soil. The cells contiguous 



