1897] THE CURVATURE OF ROOTS 323 
VIIl. THE LATENT PERIOD. 
The latent period embraces the time necessary for the con- 
version of the external force into an impulse, the transmission of 
the impulse to the motor zone, and the changes in the motor 
zone necessary to exert a bending force upon the root. 
Although no special and exact measurements of the latent 
period were made in my experiments, yet it was found in plants, 
such as Pisum and Phaseolus, in which a primary medulla is 
formed and the mechanical tissues of the motor zone are thus in 
the form of a tube, the latent period was from three to five hours. 
On the other hand, in such roots as those of Zea and other 
monocotyledonous plants, in which the fibro-vascular tissue is in 
the form of a solid cylinder of less diameter than the tube in 
Phaseolus, it would present far less resistance to the action of the 
cortex. The latent period of Zea is from one to two hours. It is 
to be borne in mind that in all such observations the roots were 
under conditions which retard curvature. The latent period of 
roots in the soil must be somewhat less. Chas. Darwin notes 
distinct curvatures in the roots of many plants, in response to 
contact, in five to nine hours (4). The movement had made 
great progress (20-30°) on the lapse of this period after excita- 
tion. It is evident that wide variations will be shown in the 
length of time between excitation and reaction. 
The manner in which impulses are conducted from the sen- 
sory to the motor zone is a matter which may not be determined 
exactly. The entire mass of protoplasts between the sensory 
zone and the motor zone are in a state of intense metabolism 
and vigorous growth, and are not entirely separated by the 
_ imperfect and newly formed walls. The distance separating the 
_ two zones may be as great as I to 2™" in some roots in a state 
of very rapid elongation, while in others the two regions must 
nearly join; indeed, it is conceivable that they may overlap in 
Certain cases (see “motor zone’’). 
The determination of the method of transmission is a matter 
ce which must wait upon a pu aceneee in es of the 
‘ ae ieloey of the cell. : = 
