PHENOMENA DUE TO THE TENSION OF TISSUES. 8oi 



cause diminishes their extensibility to an increasing extent, especially when, as in 

 the xylem of the fibro-vascular bundles, the cell-walls become lignified, which renders 

 them capable of resisting extension. The more quickly, on the other hand, the thin 

 cell-walls in the pith and parenchyma generally increase in size (especially in length) 

 by superficial growth, the stronger becomes the tension of the passively stretched 

 layers of tissue. To this must be added the peculiar power of the medullary 

 cells to absorb water from the older parts with great force and rapidity, and thus 

 to maintain themselves in a state of the highest turgidity. This distends the pilh 

 independently of the superficial growth of its cell-walls, and besides influencing the 

 more slowly growing layers of tissue, also contributes to increase the superficial 

 growth of the cell-walls of the pith. If the woody bundles then become lignified as 

 the tissues become more developed internally, and the resistance of the epidermis, 

 which is constantly becoming more cuticularised, becomes too great, these tissues 

 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 the 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 (Fig. 478), that the turgidity of the 

 cells of the stretched 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 differences 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, t. 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 different 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 



3 F 



