PROTOPLASM AND ITS PROPERTIES 183 
194. Nature and Occurrence of Irritability in Plants.t— 
Mention has already been made of the fact that certain 
parts of plants respond to suitable st¢mult that is exciting 
Fic. 126. — Protoplasm in Ovule and Fruit of Snowberry (Symphoricarpus 
racemosus). 
A, cells from ovule, x 340; B, cells from an ovule further developed, x 340; C, D, 
cells from pulp of fruit, x 110; , nucleus ; p, protoplasm ; s, cell-sap. 
In the young and rapidly growing cells, 4 and B, 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 Schimper’s Text-Book of Botany, 
pp. 160-162 and 269-274. 
