388 VEGELABLE PHYSIOLOGY 
wall or other support to which it is clinging. The roots of 
Thesium show a similar property. When they come into 
contact with other roots growing near them they develop a 
swelling at the point 
» of contact, from which 
certain cells grow out 
and penetrate the 
host, forming haust- 
oria (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 
Fie. 157.—Thesium alpinum. Prece or a tissues (fig. 158). 
ROOT WITH SUCKER IN SECTION. «x 85. Another form of 
(After Kerner.) oo pe ‘ 2 
irritability is exhi- 
bited by many growing shoots, which is perhaps somewhat 
akin to sensitiveness to contact. It is an appreciation of 
oscillation, or shaking. If a 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 stimulus 
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 evident that growth can have nothing to do with the 
movement. In these instances we have rather to do with 
a modification of turgescence, involving a redistribution 
of the water contained in the organ. The falling of the 
leaflets and leaves of Mimosa is due to a sudden change 
