8o3 
MECHANICS OF GROWTH, 
when the cell-wall is passively distended by turgidity or by the tension of the sur- 
rounding layers of tissue, it is only enabled to deposit fresh substance in the direc- 
tion of its surface when perfectly saturated ; this does not however imply that other 
causes do not cooperate in promoting the intercalation. 
The importance of turgidity as a cause of growth may be very strikingly illus- 
trated in the case of isolated cylinders of pith, as we shall show presently. 
When, in consequence of their separation, the tissues which were in a state 
of passive tension become suddenly shorter, and the pith which was in a state of 
positive tension suddenly longer, this process must be connected with a corresponding 
change in the form of the cells ^ ; the cells which contract must at the same time 
become wider in diameter, while those of the pith which lengthen must on the con- 
trary become narrower. It is impossible however to measure directly these changes 
of diameter, which are so small that ordinary methods are inapplicable. 
It is, however, a necessary consequence of what has been said that the passive 
lengthening of the epidermal cells, &c. in a growing internode makes them narrower; 
the } oung epidermis must therefore be too narrow, besides being too short for the 
inner masses of tissue. Similarly the pith, being prevented from elongating in the 
growing internode by the surrounding layers, must in consequence have a tendency 
to enlarge transversely; besides being too long for the elongated tissues, it will 
also be too thick for them, and must have a tendency to force them apart. It 
follows therefore from the longitudinal tension which has been observed in the 
layers of tissue of a growing organ, that a transverse tension must also exist in it 
of such a nature that the outer layers are in a state of passive tension, while the 
medullary cells which are prevented from lengthening have a tendency to dilate 
transversely. 
If thick transverse sections^ from growing stems are cut radially, they gape 
open, evidently because the epidermis contracts in the peripheral direction, having 
been previously of too small circumference for the inner tissue, in other words, in 
a state of passive tension. The tendency of the medullary cells which are pre- 
vented from lengthening to become broader transversely does not appear, on the 
other hand, to be always hindered by the surrounding wood and cortical tissue, but 
often to be even promoted by them ; so that these layers of tissue which surround 
the pith grow more rapidly in the peripheral direction than does the pith itself, and 
therefore exercise a radial traction upon it. A striking proof of this phenomenon is 
afforded by the frequent formation of cavities in stems and leaf-stalks at the time 
and place where the growth in length is most rapid. The increase in thickness of 
the pith is not sufficient to fill up the space which is enclosed by the surrounding 
tissues, and which increases in size ; its cells separate in the longitudinal direction, 
and the woody cylinder remains clothed on the inside by a layer of pith, the longi- 
tudinal tension of which still continues. The existence of an outward traction upon 
^ Any considerable change in the volume of the medullary cells when isolated must not indeed 
be expected, when it is recollected that neither the water contained in the cells nor the cell-walls 
permeated with water alter their volume under the forces exerted in this case. An alteration in the 
volume of the entire pith could at most arise from a change in the size of the intercellular spaces in 
consequence of the change in form of the cells. 
^ Sachs, Experimental-Physiologic, p. 471. 
