Factors in Evolution 29 



from cell to cell, somewhat as is the case in animals, 

 where there is a specialized nervous system. Our 

 knowledge of the transmission of stimuli in plants, 

 however, is very far from complete. Of a very dif- 

 ferent nature are certain movements which are 

 purely mechanical. Thus the twisting of the awns 

 in many grasses, or in the alfilaria (Erodium), or 

 the movements of the " elaters " in Equisetum or 

 the liverworts, are purely mechanical movements 

 due to the absorption of water. 



Unicellular Plants. The simplest green plants 

 consist of a single cell, which may be motile, but 

 usually is non-motile and most often globular or 

 oval in form. The cell is usually enclosed in a 

 membrane of cellulose. The protoplast contains a 

 nucleus and one or more green corpuscles or 

 chromatophores. Such a simple green cell is able 

 to perform all the essential life functions. These 

 low plants are mostly aquatic, and with the water 

 absorbed from the medium surrounding them, there 

 are taken into the cell in solution the various food 

 constituents which the cell needs for its develop- 

 ment. Oxygen is absorbed for the respiratory proc- 

 ess, and in the chromatophores the CO 2 dissolved 

 in the water is decomposed and united with the 

 hydrogen and oxygen derived from the decomposi- 

 tion of water. Such green cells exposed to the light 

 for a short time will show in the chromatophores 

 the first visible evidence of their assimilation, or 

 photosynthesis, in the form of starch, this carbon 



