BOTANY OF THE LIVING PLANT 



trolled by the centrifugal force. The shoots grew towards the 



Keel, the roots away from it. The fact that centrifugal 



ible to control the orientation of plant-organs in these 



eriments confirms the view that under normal conditions gravity 



the determining influence, since centrifugal force resembles gravi- 



tational fofce in that both will cause internal pressures within the cells. 



Fig. 95. 



hi longitudinal section of the root cap of Roripa amphibia, showing 

 Btan h paint settled to that side of the cells which was lowermost. 73=dermato- 

 calyptn 



ue of root-cap of Hrlianthus annuus after 24 hours in a horizontal position 

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



I m igrammatii representation of the statolith cells of the root-cap of Pisum 

 • t rest ; b, 30 minutes in horizontal position ; c, J, cells fixed in inter- 

 mediate positions 



(After Nemec.) 



( toe well-known theory suggests that the sensitiveness to gravity is confined 

 to certain special cells which contain relatively heavy cell-inclusions, free 

 move in the cell : these will therefore tend to lie against the physically 

 a all. These inclusions are termed statoliths and it is suggested that 

 they normally consist oi Btaxcfa grains. It is certainly the case that many 

 plant-organs include cells containing starch grains which are free to move 

 in ] toplasm, though generally speaking their position is fixed. In 



root-, tt» b cells arc usually present in the root-cap (Fig. 95), while in stems 



