THIGMOTROPISM, CHEMOTROPISM 127 
_ of stems have also been supposed to be such “statocysts.” 
190. Thigmotropism. If a tendril be touched on one 
side by some uneven object (not by a smooth object 
like a very smooth rod or a drop of water or mercury), it 
begins to curve very soon in the direction of that object. 
At the very first this curvature, which may become 
apparent within a few seconds, is undoubtedly due to 
- changes in turgor on the two sides of the tendril, but in 
only a short time rapid growth sets in on the outside, and 
the tendril winds around the object. Soon thereafter the 
part of the tendril between the stem and the object also 
begins to coil in a double spiral, this also being due to 
unequal growth. Thigmotropism, as this phenomenon 
is called, is exhibited by tendrils and by other parts of 
plants that assume this function, such as the leaf stalk of 
Clematis, peduncles of some plants, and whole shoots, 
especially modified for this purpose, of other plants. 
Special papilla-like cells have been regarded by some 
botanists as the organs of perception. The curvature of 
roots toward or away from points of injury is possibly to 
be considered as a special form of thigmotropism. It is 
often called traumatropism. 
191. Chemotropism. The hyphae (filaments of cells) of 
many fungi and the pollen tubes of seed plants show a 
peculiar growth response to the stimulus of various 
chemical substances. Thus, many pollen grains placed 
on a piece of moist filter paper will produce tubes 
growing in any direction, but if a small crystal of cane 
sugar be placed on the paper, some kinds of pollen 
tubes will change their direction of growth and turn 
directly toward it. Fungus hyphae show similar changes 
in direction of growth when they perceive various sub- 
stances in solution. In both cases certain substances 
induce positive and others negative chemotropism. 
