IRRITABILITY; POLARITY AND CORRELATION 349 



the tip in the section where the greatest growth occurs (3-4 mm. 

 from the tip). The transmission region is consequently very 

 short, since the growing region is directly behind the receptive 

 region. 



The Statolith Theory. — Precisely how the plant is able to per- 

 ceive the stimulus of gravity, has been the subject of much re- 

 search. When organs bend under the stimulus of gravity or light, 

 there is found on the convex side (1) in- 

 creased respiration, (2) increased acidity, 

 (3) increased hydrolysis of polysaccharides, 

 and (4) increased turgor. These various , 

 changes are to be considered, however, as 

 effects or accompaniments of the bending 

 rather than as causes. 



Growth hormones have been offered by 



Loeb and others as an explanation of the K _ . „ . 



, Fig. 24.— The effective 



common tropisms; and while the hormones s ti mu lus on a root varies 



may be associated with the growth, the with the sine of angle a, 



more immediate cause of the bending is i- e the angle which the 



. root is bent from the 



now attributed to other factors. vertical. 



Nemec (1902) and Haberlandt (1903) 

 proposed that the response is called forth by movable starch 

 grains in the receptive regions. When the root is growing ver- 

 tically (normally) these starch grains or statoliths are resting on 

 the lower ends of the sensitive cells (statocysts) which contain 

 them. If the root is tilted out of this position, the statoliths 

 fall on a sensitive portion of the protoplasm not accustomed 

 to them, and the mechanism which brings about the end re- 

 sponse is put into action, resulting in the return of the stato- 

 liths to their normal position at the lower end of the cell. While 

 this theory began as a purely speculative hypothesis, much 

 work has been done to confirm it/ The sensitive regions of root 

 and stem contain cells rich in starch which meet all the require- 

 ments of the theory. In Asplenium bulbiferum the statocyst tissue 

 arises during development and then disappears parallel with the 

 geotropic sensitivity; similarly, the starch grains in the endo- 

 dermis of Tagetes disappear in the dark and also the geotropic 

 sensitivity (Zollikoffer, 1918). Prankard, who has developed the 

 original theory more than anyone else, has found that objects 

 other than starch grains may serve as statoliths. In wheat culms 



