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. 



