722 MECHANICAL LAWS OF GROWTH. 



oppose an insuperable resistance to the further distension of the pith by growth and 

 turgidity, and no further elongation of the internode is possible. The tendency of 

 the pith to expand ceases ; its cells lose their turgidity, they give off their water 

 to adjacent tissues, and become filled with air. 



According to this view, which has been fully established in the main, the actual 

 motive power of growth in internodes emerging from the bud-condition is the pith, 

 and the thin-walled parenchyma generally. It is only the force thus exercised that 

 causes the other tissues to increase in length as long as they are sufficiently 

 extensible. The extraordinary absorbent power possessed by the pith enables it 

 when growing to withdraw the water from the surrounding layers of tissue, and 

 thus prevents its cells from becoming more strongly turgid, neutralising by this 

 means one of the causes of the superficial growth of the cell-walls. It must also 

 be remembered, as has already been shown in Fig. 448, that the turgidity of the 

 cells of the dilated layers is even diminished, while that of the compressed cells 

 (in the pith) is increased by the tension ; and we consequently have here another 

 Cause of diff'erences in the superficial growth of the cell-walls. Finally,- it must 

 be borne in mind that the internodes, at least of land-plants, are exposed to 

 transpiration as soon as they emerge from the bud ; but this cause of diminished 

 turgidity will affect chiefly the epidermal cells and the subjacent layers, least of 

 all the pith. 



The great importance which is here attached to turgidity as a cause of growth 

 is justified by the fact that the growth of the internodes is at once stopped by its 

 decrease, i. e. by the withering of the shoot ; while it is promoted by its increase, 

 i. e. the growth of the shoot in water or damp air. 



The first and most efficient cause of the tension of tissues in a growing inter- 

 node is therefore the different capacity for turgidity of the diff"erent tissues ; this 

 depending partly on the nature of their fluids, partly on the structure of their cell- 

 walls, and partly on their relative position in the internode. A more secondary place 

 must be assigned to the swelling of the cell-walls caused by imbibition ; since it may 

 be assumed that even when the turgidity of the cell is slight, the cell-wall still obtains 

 sufficient water to satisfy its capacity for imbibition. If it were directly dependent on 

 this, all the layers of tissue would grow equally rapidly, even when the turgidity was 

 small, or had entirely disappeared. I rather hold the state of the case to be that 

 when the cell-wall is passively distended by turgidity or by the tension of the sur- 

 rounding layers of tissues, 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 posi- 

 tive 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 



^ Any considerable change in the vohime 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 



