400 VEGETABLE PHYSIOLOGY 



by a peculiar churning movement of the protoplasm in the 

 cells upon the side which becomes concave. This move- 

 ment, which Darwin, who discovered it, called aggregation, 

 is attended by a loss of turgidity. 



MOISTURE. Sensitiveness to variations in the moisture 

 of the environment is not so widely distributed as are the 

 forms of irritability hitherto discussed. It is exhibited 

 among green plants chiefly by young roots and by the 

 Thizoids of the Hepaticce ; it also occurs in the hyphaB of 

 certain Fungi. These organs tend to curve in the direction 

 of a moist surface if they are growing near one. When 

 young seedlings are cultivated in a vessel which contains 

 moist sawdust or sand, and is perforated so as to allow 

 the rootlets to protrude, these at first grow vertically down- 

 wards, according to their geotropism. Soon after they 

 protrude they curve to a greater or less extent towards the 

 moist surface, as if seeking the moisture. This behaviour 

 can be seen more easily if the vessel is inclined at an angle 

 to the vertical. The phenomenon is known as Jiydrotropism. 

 The root-tip, as in other cases, is the sensitive part ; while 

 the curvature takes place further back, where growth is 

 most active. Negative hydrotropism or aphydrotropism is 

 very rare, being exhibited only by some of the Myxomycetes, 

 which move away from moisture. 



The advantage of this form of sensitivity is evident in 

 the case of the root, which by virtue of it is drawn towards 

 the moisture of the soil as it penetrates between its particles. 



A curious instance of appreciation of lack of moisture 

 is afforded by Porlieria hygrometrica, which under such 

 conditions closes its leaflets much as nyctitropic plants do 

 when light gives place to darkness. 



CHEMICAL STIMULI. We have already alluded to the 

 fact that the various metabolic phenomena of plants are 

 influenced very considerably by changes in the composition 

 of the sap which the cells contain ; that certain constituents 

 stimulate the protoplasts to initiate or to alter particular 

 reactions in those cells. Besides these responses to chemical 



