316 BOTANICAL GAZETTE [MAY 
occurrence of possible stimuli, modified to some extent by the 
character and inclusiveness of the reaction. 
Although the motor zones of the shoots do not include as 
large proportions of the plant as the sensory zone, yet the dis- 
tribution is fairly general throughout the growing regions. It 
is possible to induce curvatures in some stems in which growth 
has almost entirely ceased. The curvature, however, is accom- 
panied by a revival of the growth activity. 
The functions of the root are not so numerous as those of 
the shoot, and while the efficient performance of the necessary 
amount of absorption to keep pace with the increase in mass 
and surface of the shoot has demanded a repeated branching, 
yet no segmentation like that of the shoot has occurred. The 
secondary function of the root, fixation, is purely mechanical, and 
the separation of the two functions has not been effected by a 
localization of the functions in different organs, but is an inci- 
dent to the stage or degree of development of these organs. 
Physiologically the basal portion of roots sustains a relation to 
the absorptive system similar to that of the basal portions of 
typical stems to the chlorophyll bearing and reproductive organs. 
In the earlier stages of growth any given portion of the root 
is purely directive, next absorptive, and in later periods is exclu- 
sively fixative. Only in certain special classes of aerial and 
other plants does a separation or isolation occur. The stem, on 
the other hand, is at first directive, and then fixative, and does 
not in any stage of its existence assume the relative importance 
which is to be ascribed to every portion of the root in one 
period of its development. 
In explanation of this different method of development it is to 
be said that the roots have always been surrounded by much more 
uniform conditions in time than the shoot, and in consequence 
have met the necessity for a much narrower range of adaptive 
modifications. But while the range and rapidity of variation of 
outward conditions affecting the roots have been much less than 
those of the shoot, yet the inequalities of diffusion and distri 
bution of the nutritive factors are much greater than those 
