318 BOTANICAL GAZETTE [May 
morphological characters can be assigned to the various organs 
of irritability. As a matter of fact the differentiation is entirely 
physiological, as it will be, indeed, in all organs in which the 
irritability is only a temporary character. It is therefore impos- 
sible to do more than to determine the relative position of the 
masses of cells in which in turn the various parts of this com- 
plex function are located. 
VI. FORCES ACTING AS STIMULI IN ROOTS. 
In accordance with the above it is found that the roots react to 
geotropic, heliotropic, thermotropic, hydrotropic, galvanotropic, 
rheotropic, chemotropic, and traumatropic stimuli, besides 
exhibiting rectipetality or autotropism. These terms are used 
in an inclusive sense, without reference to the phase of reaction 
under each form. Under traumatropism are included all of the 
reactions to mechanical stimuli, resulting in contact or injury, as 
well as the action of corrosive chemicals. It is to be noted that 
many roots do not exhibit all of the forms of irritability enu- 
merated. 
In the study of the mechanism of curvatures which forms a 
part of this paper I have examined geotropic, rheotropic, and 
traumatropic curvatures, and since no essential difference could 
be detected, chief attention was paid to curvatures obtained by 
geotropism. 
VII. THE SENSORY ZONE. 
The history of the researches bearing upon the location of 
the sensitive tissue of the root is a long one, and begins with 
Darwin’s experiments in which decapitated roots were found to 
be incapable of response to the forces to which they usually agi 
(4). The pathological condition induced by the decapitation 
made the conclusion that the sensitive tissue was located in the 
_ extreme apex unsafe, and it was bitterly opposed by Sachs (28), 
_ Detlefsen, and others, and it was not entirely determined beyond 
doubt until the recent brilliant experiments of Pfeffer (21): in 
which it was shown that if a root were forced to grow in a Del 
tas 
