PLANT PITYSIOLOGY 149 
mechanical contact. It must be clearly understood that 
a plant will only exhibit such a response to stimulus, if 
it is ina healthy condition, and if it has all the requisites 
for healthy growth, i.e. a suitable temperature, moisture, 
etc. 
124. Light.—The influence of Light on plants may be 
considered from two points of view,—(L) its stimulating 
action, (2) its directive action. ; 
125. The Paratonic Influence of Light is the term 
used to denote the effects produced in plants by varia- 
tions in intensity of light. The general influence of 
light in this respect is to retard growth. Plants grow 
more quickly in the dark. On the other hand, entire 
absence of light would, in the end, cause the plant to 
become unhealthy, the stems being white, and the 
leaves yellow (the etiolated condition). 
Movements are induced in some plants by variations 
in the intensity of light. Of this class are the “ Sleep- 
movements ’’ of many leaves, e.g. Cassia, Oxalis and 
Mimosa pudica, which droop and close up at night. 
The same effect may be produced by too great illu- 
mination. In both cases the use of the movement is 
to prevent excessive transpiration. 
Some flowers show movement induced by variations 
in the intensity of light. Mesembryanthemum closes 
its flowers at sunset, so does Portulaca, the Four-o’clock. 
Others like Oenothera, the Evening Primrose, open at 
night. This movement is caused by unequal growth in 
the lower and upper surfaces of the floral leaves. 
126. The Heliotropic Influence of Light.—This is the 
name given to the influence of light on the dircction of 
