STIMULATION AND ITS RESULTS 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 Hepatice ; it also occurs in the hyphe 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 Miclined 
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 Myzxomycetes, 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 stimuL1.—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 
