358 BOTANICAL GAZETTE [May 
changes of elastic extensibility, which finally becomes converted 
into ductility. The last method has been somewhat conclusively 
demonstrated by Noll in stems (16). The chief evidence 
upon which this conclusion rests consists in the fact that the 
epidermal and collenchyma cells of the convex side show an 
enlargement of three diameters during curvature, and that the 
enlargement is accompanied by a decrease in the thickness of the 
cell walls. Not only are the membranes of the convex side thinner 
than those of the concave side, but they are thinner than those 
of normal tissues of the same stage of development. The 
extension is also accompanied by changes in the qualities of the 
membranes, as shown by refraction and reaction of staining 
fluids. 
In the application of the same tests to the curvatures of roots 
some difficulty is encountered on account of the relatively small 
thickness of the walls; furthermore, the different condition of 
the tissues must be taken into account. In stems the epidermis 
and collenchyma are in a state of active growth which may be 
maintained for a long period, and these layers may elongate 
during curvature with a rapidity equal to that of the cortex, and 
they may not; in the latter instance they will experience stretch- 
ing tension fromthe cortex. In roots, on the other hand, the 
epidermal and sub-epidermal layers are not in a state of rapid 
elongation, but have attained the greater part of their growth ; 
furthermore, these cells are capable of active enlargement during 
a period of one or two days at most, and are then cast away. In 
consequence of this fact the peripheral layers of cells undergo 4 
passive stretching on the convex side which increases the axial 
and decreases the radial diameter. The reverse is true of the 
concave side. The underlying layers of cortex in Zea undergo 
an axial extension in the convex side, and a radial extension of 
the concave side. Alterations in the radial diameter of the first 
and the axial diameter of the second are not exactly ascertained, 
but the amount of change must be very slight. The roots of 
Phoenix have a much greater relative thickness than those of 
Zea, and are furnished with a layer of sclerenchymatous tissue 
