314 BOTANICAL GAZETTE [MAY 
amount is a fundamental necessity of every organism, and to the 
conditions attendant upon the performance of the nutritive func- 
tions must be ascribed the chief causes underlying the develop- 
ment of the plant body. The chlorophyll processes, therefore, 
have been the paramount factors in the development of the 
shoot, and the necessities attendant upon their proper perform- 
ance will account for the method of differentiation of the shoot, 
and the very great degree of segmentation and branching which 
it has attained. The segmentation of the shoot has made possi- 
ble not only the profitable display of ever increasing areas of 
chlorophyll bearing tissues, the proper elevation, orientation, and 
isolation of the reproductive organs, but also a separation of the 
minor functions and the differentiation of special organs for 
their performance. The separation of nutritive, reproductive, 
and other functions has been accompanied by a contempora- 
neous separation and development of the special forms of irrita- 
bility which are concerned with the forces dealt with by each 
organ. Thus, for example, the most important factor in the 
processes carried on by the leaf is the radiant energy derived 
from the sun. As a necessary concomitant of the advantageous 
use of this energy, the leaf has developed a strongly marked 
irritability to light and heat rays, and, as a result of the relations 
of the organ to the horizon in response to its heliotropism and 
thermotropism, it has acquired in some instances also a trace of 
geotropism. 
In the accomplishment of the reproductive cee an 
incidental condition is the transference of the pollen from its 
place of formation to the surface of the stigma in the same OF 
other flowers. In a great majority of instances the relation of 
ve the line j joining the anther and the stigma to the vertex or hori- 
zon is of the utmost importance, whether the pollination is” 
accomplished by insects or automatically by air currents, and a 
_ well marked geotropic reaction is therefore generally exhibited 
? by flowers th the camara a located in the a These : 
tropic reactions. be 
“The same process - _aaalyoie may be © applied to the entire . 
