MODIFICATION OF GROWTH CAUSED BY PRESSURE AND TRACTION. 809 
Sect. 16. — Modification of Growth caused by Pressure and Traction. 
Cells or whole masses of tissue may be subjected to pressure and traction in very 
different ways. On the one hand these forces may result, in a perfectly normal 
manner, from the tension of the tissues ; on the other hand, external and more 
accidental circumstances may cause single cells or masses of tissue to be com- 
pressed or stretched in particular places by solid bodies, or tissues to become 
accidentally freed from the pressure and traction to which they are normally subject. 
The numerous phenomena which indicate or prove that growth is altered in this 
way have however at present been exactly investigated from this point of view in 
only a few cases. The following will therefore only serve to draw attention to a 
subject further discoveries in which will doubtless contribute largely to the establish- 
ment of a mechanical theory of growth. 
I. Every cell-wall is subject to Pressure from within, by which it is distended, 
so long as the cell is turgid. But since the daily experience of microscopists 
teaches us that all growing cells are turgid ; and that no cell which is unable to be- 
come turgid in consequence of openings in its cell-walls has any power of growth ; 
and that withered internodes, leaves, and roots do not grow, while these organs 
grow more rapidly the more strongly turgid they are, it may be inferred that turgidity 
is an essential condition of the growth of the cell-walP. This appears to a certain 
extent intelligible if Nageli's theory of growth and Traube's experiments on artificial 
cells described in Sect, i of Book III are accepted. It may then be assumed that 
the interstices between the solid particles of the cell-wall which are occupied by 
water increase slightly in consequence of the distension of the cell-wall caused by 
the hydrostatic pressure of the sap ; and that space is thus obtained for the interca- 
lation of fresh particles of soHd substance ; the distension caused by turgidity then 
begins afresh and produces the same effect. 
The distension which takes place at any particular spot of the cell-wall and the 
consequent intercalation of fresh solid substance depend however chiefly on the 
internal properties of the cell-wall itself. Not only do different parts of the cell- 
wall differ in their extensibility, but they may even vary at the same spot in this 
respect in the longitudinal and in the tangential or the oblique direction, as may 
be seen from the swelling of the cell-wall. But that there is actually such a general 
difference in the extensibility in different directions is at once shown by the fact 
that growing cells assume the most various forms, — cylindrical, stellate, &c. ; while, 
if the extensibility of the cell-wall were the same in all directions, the cells must all 
become spherical as the result of turgidity, or polyhedral under that of mutual 
pressure. This little is nearly all that we know at present with reference to exten- 
sibility, turgidity, and growth by intussusception. It must be borne in mind that 
tägliche Periodicität der Dickendimensionen der Baumstämme, Diss. Inaug., Halle 1879). See also 
Kraus, Ueb. Wasservertheilung in der Pflanze, III, Die tägliche Schwellungsperiode der Pflanzen, 
Halle 1881.] 
' [De Vries has shown (Mechan. Ursachen d. Zellstreckung) that when a growing internode is 
placed in a 5-10 per cent, solution of a neutral salt, its cells lose their turgidity, and that growth 
ceases; when the salt solution is washed out with distilled water, the cells regain their turgidity and 
growth is resumed. Sorauer (Bot. Zeitg. 1878) has pointed out that individuals grown in dry air 
are much smaller than others of the same species grown in moist air. These facts, and many others 
which might be mentioned, show the dependence of growth upon turgidity.] 
