STIMULATION AND ITS RESULTS B97 
with other roots growing near them they develop a swell- 
ing at the point of contact, from which certain cells grow 
out and penetrate the host, forming haustoria (fig. 157). 
The parasite Cuscuta, often found growing on clover, is 
affected in the same way, first twining round the clover 
stem and then putting out haustoria, which penetrate its 
tissues (fig. 158). 
Another form of irritability is exhibited by many 
Fie. 157.—Thesium alpinum. Precz or a Root witn Sucker IN 
Srzcrion. x 35. (After Kerner.) 
growing shoots, which is perhaps somewhat akin to sensi- 
tiveness to contact. It is an appreciation of oscillation 
or shaking. Ifa shoot is gently struck laterally several 
times near its base, its apex -curves over towards the 
side struck. If the blows are given near the apex, the 
resulting curvature is in the opposite direction. If a plant 
of Mimosa pudica is shaken, the leaves fall as they do when 
they are violently handled. 
The mechanism whereby the response to the sieatiiins of 
contact is brought about in growing organs we have seen 
to be an increased turgidity on the convex side, followed 
by growth. In those cases where the organ is mature it is 
