28 Plant Life and Evolution 



living slime responds promptly to the various stimuli 

 which may be brought to bear upon it. 



As a rule, however, the protoplasm of plants is 

 shut up in closed cells, but the living protoplast 

 included within the cells often shows evident move- 

 ments, and may be seen to react toward stimuli in 

 the same way as the naked plasmodium of the 

 slime-mold. If the green corpuscles or chloroplasts 

 are present in the cells, these may be seen to shift 

 their position under the influence of light, and the 

 protoplasmic movements which are very common 

 within the cells are affected readily by different 

 stimuli. When the protoplasm escapes from the 

 cell, as it sometimes does in the reproductive cells, 

 especially among the lower plants, these motile cells 

 usually react promptly to various stimuli. Thus 

 zoospores will usually swim towards the source of 

 light, and the spermatozoids of ferns are strongly 

 attracted by the salts/ of malic acid. 



Reactions of Multicellular Organs, Due to Irri- 

 tability of Protoplasm. The movements and other 

 indications of response to stimuli, shown by the 

 multicellular organs of the higher plants, are un- 

 doubtedly induced primarily by the reaction of the 

 protoplasm within their cells. It has been shown 

 that in many plants there is a direct communication 

 between the protoplasts of neighboring cells, due 

 to the penetration of the cell walls by the fine 

 threads of protoplasm, and it is highly probable that 

 in this way the effects of stimuli may be propagated 



