28 ; ' - é ly di $ 
198 = Structural and Physiological Botany. 
direction of the axis of the growing structure, and are therefore called 
longitudinal tensions. But when, after growth in length has ceased, a 
permanent increase in thickness takes place, as for instance in the case 
of the stems of forest-trees, then this increase in thickness is accompanied 
by a transverse tension in the radial and tangential directions. This is 
generally caused by the cortical tissue growing more slowly than the 
wood; and this, as has already been shown, causes the bark to become 
too small for the wood, and hence to compress it considerably ; no in- 
considerable pressure being exerted, on the other hand, by the wood on 
the bark. This can be easily shown by stripping a ring of bark from a 
stem, and slitting it up one side, when it contracts, and will no longer 
enclose the wood, its edges gaping apart. 
Tensions of different layers do not occur only in tissues, but also in 
the cell-walls of individual cells ; and it is then usually the outermost 
layers of the cell-wall which are stretched by the inner ones, while 
these latter are correspondingly compressed by the elasticity of the outer 
layers. 
The turgidity of the cell, or the pressure exercised by the cell-sap 
on the enveloping cell-wall, must be carefully distinguished from the 
tension of the layers of tissue. Turgidity is caused by the substances 
dissolved in the cell-sap attracting the water which surrounds the 
cell by the force of endosmose ; the water accumulates within the cell- 
cavity by endosmotic attraction, and, exerting a pxessure on the inside of 
the cell-wall, places it in a condition of greater or less tension. 
The final results of the growth of the cell-wall caused by the various 
forces enumerated—its extensibility and elasticity, and the turgidity of 
the cell—is a measure of the rigidity or flaccidity of the plant or part of 
the plant. The total tension, for instance, is diminished, and the part 
becomes more flexible, when the turgidity decreases from loss of water, 
or when the elasticity of the stretched cell-walls diminishes, or when the 
cell-walls become more extensible, or, finally, when the inequality of the - 
growth of the different layers of tissue diminishes. When a change 
of this kind takes place on all sides, the part of the plant will be- 
come shorter or longer according ‘as the tension diminishes or increases ; 
but if it takes place on one side only, a corresponding curvature is 
caused. 
To complete the account of the forces at work in the plant, we must 
add to those already mentioned, which are peculiar to the molecular 
structure of their organised parts and to the phenomena of tension, 
others which are set free by the movement of water and of gases within 
the plant. Sufficient reference has already been made to the former 
under the head of nutrition. The latter class of movements is easily 
