GROWTH AND MOVEMENT 



465 



were displaced, the granules would settle upon a new and excitable side 

 of the vacuole wall, starting into action the mechanism of the end re- 

 sponse. 



There are many objections to this form of the theory, which was suggested by tlic 

 visible otocysts of Crustacea, and the appearance of the centrosomes, which were 

 then supposed to be common in the cells of seed plants. 



In a more concrete form the theory has much to commend it, though 

 it cannot yet be considered as firmly established. In this form no in- 

 visible structures are predicated, but the principle is the same. Certain 



Figs. 697, 698. — Perceptive regions: 697, median longitudinal section of the rootcap 

 of Roripa amphibia ; d, dermatogen; 698, apex of the coleoptile of the plumule of 

 Panicum miliaceum. — After Nemec. 



cells, notably those of the inner median portions of the root cap (fig. 697), 

 the tip of the coleoptile in grasses (fig. 6g8), and a layer around the vas- 

 cular cyhnder in stems, contain rather large starch grains in such abun- 

 dance as to attract attention. Moreover, these starch grains are freely 

 movable, and in whatever position the organ rests, they accumulate on 

 the physically lower side of the cells. They seemed to answer the re- 

 quirement for bodies heavier than the fluid in which they lie, and there- 

 fore capable of setting up an excitation by coming to rest upon a part of 

 the protoplast unaccustomed to their contact. It is assumed that cer- 

 tain areas of the protoplast are properly sensitive; that their excitation 

 will start into activity the mechanism of curvature, which will eventually 

 restore the organ to its normal position and so remove the irritating starch 



