73 O MECHANICAL LAWS OF GROWTH. 



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-wall. 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 intercalation of fresh particles of 

 solid substance ; the distension caused by turgidity then begins afresh and pro- 

 duces 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 the rapidity of the growth of cells is in proportion to the thinness and 

 therefore the extensibility of their walls. The growth in thickness of the cell-wall 

 usually begins when the increase of the cell in volume begins to diminish or has 

 altogether ceased. 



If then the distension of the cell-wall caused by turgidity is the origin of its 

 superficial growth, something similar must also occur when the cell-wall is stretched 

 in some other way by external forces, the turgidity being less. This is the case 

 with the epidermis and cortex of shoots as a result of the tension of the tissues. 

 Since in long internodes and leaves these cells usually grow principally in the longi- 

 tudinal direction, while in broad leaf-blades they assume the form of polygonal 

 plates, this may be referred in the first case partly to the disturbance to which 

 they are subject being chiefly in the longitudinal direction, in the second case to its 

 being in all directions parallel with the surface \ It has already been stated that 

 the cells of the primary cortex of shoots which are increasing rapidly in thickness 

 are not merely stretched but also grow rapidly in the tangential direction^. 



2. Pressure from ivitJiout on the ceU-ivaU which is distended by turgidity occurs 

 in a very simple form when the apices of growing cells come into contact with solid 

 bodies; as the root-hairs of land-plants with the particles of the soiP. The very thin 



* For further details on the possible influence of tension on the formation of stomata, see 

 pfitzer, Jahrb. fiir wiss. Bot. vol. VII, p, 542. 



2 On the connection of the radial and peripheral arrangement of rows of cells in a transverse 

 segment with the increase in diameter, see the lucid description of Nageli in his Dickenwachslhum 



des Stengels bei den Sapindaceen, Munich 1864, p. 13 et ieq. 

 Sachs, Experimental-Physiologic, p. 186. 



