362 BOTANICAL GAZETTE [MAY 
material. The weight of evidence obtained by Noll and myself 
is in favor of the first named method. 
The exact region of the motor zone which is set in activity 
by the impulse from the sensory zone embraces a part of the 
cortex consisting of the fourth to the eighth layer of the cortex 
in Zea, and from the fifth to the tenth or eleventh layer in 
Phoenix. The changes consequent upon a reception of an 
impulse occur in the walls of these cells only, and their active 
extension results in the stretching of the external or peripheral | 
layers. 
It must be supposed that the increase in elasticity extends to 
the radial walls in Phoenix. The folding of the walls of the 
motor cells of roots is doubtless due to the great resistance to 
their expansion offered by the peripheral layers. Marked or 
sharply folded walls are not to be found in the convex sides of 
stems and other organs in which all of the tissues are more OF 
less active in the elongation. ‘ 
The comparatively great radial growth of the epidermal cells 
of the concave side subsequent to curvature must be taken as a 
consequence of the mechanical strain exerted upon this layer. 
XV. RECAPITULATION. 
The contents of the foregoing paper may be summarized 
briefly in the following paragraphs: 
1. In order to determine the nature and mechanism of a cur- 
vature, the phylogenetic meaning and purpose of the movement, 
the arrangement of the mechanical tissues, and the stage ofS 
development of the organ must be taken into consideration. — 
The curvatures of stems are not identical with those of most 
tendrils, or of many roots. ‘ 
_ 2. It has been established beyond all doubt, by previous 
investigations, that curvatures are due to changes in the cell wall, 
rather than in the osmotic activity of the cell contents. The 
only determination of the real nature of curvature is to be 
accomplished by an anatomical examination of the cells of the 
motor zone before, during, and after curvature has taken place. 
