46 CJiapters in Modern Botany chap. 



even a minute trace of certain fluids, especially nitro- 

 genous, acts as a stimulus. 



During the bending of the tentacle the secretion of 

 the gland becomes more copious, and its chemical reac- 

 tion changes from neutral to acid. Meantime within the 

 stalk of the stimulated tentacle a strange change occurs, 

 marked externally by a somewhat mottled appearance. 

 When examined under the microscope the formerly homo- 

 geneous fluid contents of the cells of the stalk are seen 

 to have separated in purple bead-like masses, of constantly 

 varying number, shape, and size, and suspended in a 

 colourless fluid. This change makes the layer of colour- 

 less circulating protoplasm which lines the cells more 

 distinctly visible. 



Darwin attached considerable importance to this process, 

 which he termed "aggregation of the protoplasm." It 

 begins in the glands, and gradually travels down the 

 tentacle from cell to cell. After the action of the tentacle 

 is over, a reverse process of redissolution of the protoplasm 

 proceeds from the base upwards. Darwin believed that 

 it was a vital process, only exhibited when the cells were 

 alive and normal, not necessarily connected with either 

 bending or increased secretion, and quite different from 

 " plasmolysis " or the shrinking of the protoplasm from the 

 cell -wall, which is observed when parts of plants are 

 examined in any dense fluid which induces osmosis. 

 Darwin observed a similar aggregation in the sensitive 

 hairs of Dionaea, and in the roots of various plants, and 

 believed that it was of wide occurrence and of profound 

 importance in the physiology of the vegetable cell. 



In connection with the sensitiveness of the Drosera, 

 one of the most interesting of Darwin's observations was 

 in regard to the salts of ammonia. All the salts of 

 ammonia cause the tentacles to bend — the carbonate 



