1897] THE CURVATURE OF ROOTS 363 
3. The development and organization of irritability in roots 
and shoots has been widely different. The segmentation and 
branching of the shoot, in order to facilitate food formation and 
reproduction, has been accompanied by an isolation and separa- 
tion of the forms of irritability, a great extension of the sensory 
surfaces, and a less widely extended distribution of motor 
regions. The development of the root in order to facilitate 
absorption has resulted in a coincidence of many forms of irrita- 
bility, both as to sensory and motor regions in the extreme 
apex of the growing organ which undergo branching but no 
segmentation. 
4. The organs of the irritable mechanism of roots exhibit a 
physiological rather than a morphological differentiation. 
5. The sensory zone. The mass of protoplasts of the root 
capable of converting certain external forces into forms of energy 
which induce movement constitutes the sensory zone. The term 
“perceptive zone” has hitherto been improperly applied to this 
Tegion. Roots exhibit reaction to injuries which cut away a thin 
slice of the periblem, and to incisions in the periblem which do 
not affect the punctum vegetationis, as well as to incisions which 
cut away the punctum vegetationis entirely. Furthermore, injuries 
directly apical and affecting the punctum vegetationis alone do 
not cause reaction, and it is probable that the punctum vegetationis 
does not form an essential part of the sensory zone. The sensory 
zone, therefore, consists of a cup-shaped mass of periblem 
extending 1 to 2™ axially, from which the bottom, represented 
by the punctum vegetationis,is lacking. The sensory zone extends 
4pproximately to the forward edge of the motor zone. 
6. Transmission of impulses and latent period. The latent 
Period of the reactions of roots varies from one to fifteen hours 
according to the nature of the stimulus and the mechanical 
- qualities of the root. The latent period of geotropic reactions 
of Zea may be no more than one hour, of traumatropic reactions 
_ ten hours. The contiguity of the sensory and motor zones ren- 
_ ders no special provision for the transmission of impulses neces- 
__ Sary, and leads to the conclusion that the greater portion of the 
