THE MOVEMENTS AND SENSITIVITY OF PLANTS 327 



zontally, having lost the capacity to perceive the direction of 

 growth. In shoots of small grains, geotropic sensitivity is cen- 

 tered in the plumule, while in the majority of stems it is more or 

 less equally distributed throughout the growing zone. 



The question as to exactly how plants perceive the direction 

 of gravity is of great interest, but can by no means be regarded as 

 finally solved. The theory advanced by two scientists, Nemec 

 and Haberlandt, seems to have been popular for a long time. 

 According to this theory, the perception of the direction of gravity 



Fiu. 134. — Statoliths in the root tip {after 

 Haberlandt) . 



Fig. 135. — Statoliths in the cells of 

 the plumule of millet {after Haber- 

 landt). 



is realized by means of mobile grains of starch found in certain 

 cells. Having a relatively high specific gravity, the starch grains 

 fall to that side of the cell which happens to be lowest and thus 

 produce a pressure on the layer of protoplasm covering the wall of 

 the cell. These mobile grains of starch are called "statoliths." 

 Their greatest accumulation has been observed in the cells of the 

 root cap (Fig. 134) and the tip of the plumule of millet (Fig. 135). 

 In the stems of most plants, similar grains may be found in the 

 cells of the so-called starch sheath or endoderm, which forms a 

 cylindrical layer in the stem. This distribution of statoliths in 

 certain parts of a plant is quite in accordance with the fact that 

 in the shoots of cereals and in roots geotropic sensitivity is cen- 

 tered in the tips, while in stems it is spread throughout their length. 

 As a result of the discovery of new facts on the hormone nature 

 of phototropism (Art. 94), the causes of geotropic curvature of 



