1897 ] THE CURVATURE OF ROOTS 319 
tube in such a manner that the section I to 2™™ in length, includ- 
ing the punctum vegetationis, assumed a position at right angles to 
the axis of the basal portion, and then placed in such position 
that the bent apex was in a position of equilibrium, no excita- 
tion occurred. A concise history of the various researches deal- 
ing with the localization of the sensitive tissue, previous to the 
experiments of Pfeffer, was given by Rothert in 1894 (21). 
The result of all the investigations upon the matter shows 
that the mass of sensitive tissue is located in the peripheral por- 
tion of the punctum vegetationis. The excision of a mass of cells 
not exceeding .5“™" removes this zone of sensitive tissue 
entirely from the roots of Za mazs, and since the penetration of 
the growing zone beyond the outer layers produces other effects 
besides those due to irritability, it may be assumed that the 
Sensitive tissue is in the form of a cup with walls consisting of a 
few layers of cells only. Furthermore, the cells of the walls of 
the cup acquire the special power of reception of outward stimuli 
Shortly after their formation, and retain it for a short time only, 
during which time the punctum vegetationis moves forward 
and forms new layers in front of them. This period in most 
roots extends over a few hours only. After this time, these cells 
lose the power of corverting impinging forces into impulses, and 
retain only the primitive forms common to all. Whether or not 
this specialized mass of cells, or rather the cells in the special- 
ized stage, are arranged as a complex organ, in which the indi- 
vidual and separate action of the cells is necessary, or whether 
each individual cell is capable of giving rise to the force consti- 
tuting an impulse, has not been ascertained, since insurmounta- 
ble technical difficulties stand in the way of a determination of 
the matter. It appears more likely, however, that the concerted 
_ and organized action of a number of the protoplasts of the irrita- 
ble zone is indispensable, especially in such reactions as those of 
geotropism. 
__ This conclusion is favored by results obtained by Spalding in 
his study of traumatropic curvatures (28). He says: Ce 
Tt soon became evident that the nature, direction, and extent of the wound 
