STIMULATION AND ITS EESULTS 391 



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 

 rhizoids of the Hepaticce ; it also occurs in the hyphae of 

 certain Fungi. These organs tend to curve in the direc- 

 tion 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 

 downwards, 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 hydrotropism. 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 sensibility 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 con- 

 stituents stimulate the protoplasts to initiate or to alter 

 particular reactions in those cells. Besides these responses 

 to chemical stimuli there is evidence that vegetable 

 protoplasm can modify its normal behaviour in other 

 ways when exposed to similar influences. This form of 



