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 
rhizoids of the Hepatice ; it also occurs in the hyphe 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 gand, 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 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 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 Porheria hygrometrica, which under such 
conditions closes its leaflets much as nyctitropic plants do 
when light gives place to darkness. 
Cuemican Srrmut1—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 gap 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 
