126 



BOTANY OF THE LIVING PLANT 



stem. If the fact be as suggested, then the movements of geotropism provide 

 an example in Plants of Stimulus and Reaction. Further, there is a localisa- 

 tion of the reception of the stimulus, and also of the conveyance of that 

 stimulus from the point where it is received to the point where reaction 

 appears. The analogies with the behaviour in the Animal body are obvious. 



FIG. 87 bis. 



A. Median longitudinal section of the root-cap of Roripa amphibia, showing 

 starch-grains settled to that side of the cells which was lowermost. D = dermato- 

 calyptrogen. 



B. Tissue of root-cap of Helianthus annuus after 24 hours in a horizontal position. 

 Arrow (0) shows axis of root. (S) shows direction of gravity. 



C. Diagrammatic representation of the perceptive cells of the root-cap of Pisum 

 sativum. a, at rest ; b, 30 minutes in horizontal position ; c, d, in course of reaction, 

 to show the shifting of the aggregation of protoplasm. 



(After Nemec.) 



Even the use of ponderable solid starch-grains has its parallel in the otoliths 

 of the Crustacea. But whatever opinion may be held as to the validity of this 

 theory of perception, finally the phenomenon of response is relegated back to 

 the living protoplast, to which the ponderable bodies are merely the means 

 of conveyance of the stimulus of direction. 



It is a familiar fact that when Plants are grown in a room having 

 a window lighting it on one side, the leaves and shoots grow towards 



