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 the 
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. 698), and a layer around the vas- 
cular cylinder 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 
