PROTOPLASM AND ITS PROPERTIES 



183 



194. Nature and Occurrence of Irritability in Plants.' — 

 Mention has already been made of the fact that certain 

 parts of plants respond to suitable stimuli that is exciting 



Fio. 126. —Protoplasm in Ovule and Fruit of Snowberry (Symphoricarpus 



racemosus). 



A, cells from ovule, x 340 j B, cells from an ovule further developed, x 340 ; C, 2), 



cells from pulp of fruit, x 110 ; «, nucleus ; p, protoplasm ; s, cell-sap. 

 In the young and rapidly growing cells, A and J3, the cell-sap is not present, or 



present only in'Small quantities, while in the older cells, C and D, it occupies 



a large portion of the interior of the cell. 



causes. Geotropic movements (Sect. 70) are due to 

 the response of roots or shoots to gravitation. These 



1 See Strasburger, Noll, Schenk, and Sohimper's TextSook of Botany, 

 pp. 160-162 and 269-274. 



