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 pith 
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, 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 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 
