1897 ] THE CURVATURE OF ROOTS 321 
point, and that this tissue is in the form of a cone shaped cup, 
the rounded bottom of which is extremely thin, or is wholly 
absent. 
As a matter of fact, it appears from the results at hand that 
the punctum vegetationis does not form a part of the sensory zone. 
Some further investigations upon this point are in progress. It 
is highly improbable that the growing point, shielded by the 
thick root cap, should have acquired any special irritability to 
Sa 
Fic. 3 FIG. 4 
FIG. 3. Diagram showing extent of excision producing curvature in a root 
of Phoenix dactylifera. 
FIG. 4. Diagram showing location and extent of incision producing curva- 
ture in a root of Arisaema triphyllum. 
external forces, particularly of a mechanical nature. It is to be 
remembered that the specialized receptive zone of the root tip is 
a physiological, not a morphological differentiation. This zone 
resides in the embryonic tissue during a limited number of hours 
only, and moves steadily forward. Furthermore, this single zone 
is capable of the reception of stimuli of all the classes to which 
the root as a whole reacts, eight in number. It is to be noted 
that in irritable mechanisms of such character the phenomena of 
accommodation may not occur. The residence of the special 
forms of irritability is too brief to permit the protoplasm to 
recover from continued stimuli. In the root the period of irrita- 
bility is but little greater than the latent. period. 
_ This region, capable of receiving special stimuli and origina- 
_ting motor impulses, has been termed the perceptive cone. I am 
unable to trace such an application of the term to its origin, but 
_ find that it has been in use in the publications of the botanical 
