THE FORMATION OF WOOD IN PLANTS. 413 
of the water will be sent into the blotting-paper; at each relaxation the sponge will refill 
from below, to give another portion of its contents to the blotting-paper when again 
squeezed. 
But how does this explanation apply to roots? If the formation of wood is due to 
intermittent transverse strains, such as are produced in the aérial parts of upright plants 
by the wind, how does it happen that woody matter is deposited in roots, where there are 
no lateral oscillations, no transverse strains? The answer is, that longitudinal strains 
also are capable of causing the effects described. It is true that perfectly straight fibres 
united into a bundle and pulled lengthways would not exert on one another any lateral 
pressure, and would not laterally compress any similarly straight canals running along 
with them. But if the fibres united into a bundle are variously bent or twisted, they | 
cannot be longitudinally strained without compressing one another and structures im- 
bedded in them. It needs but to watch a wet rope drawn tight by a capstan, to see that 
an action like that which squeezes the water out of its strands, will squeeze the sap out 
of the vessels of a root into the surrounding tissue, as often as the root is pulled by the 
swaying of the plant it belongs to. Here, too, as before, the vessels will refill when the 
pull intermits; and so, in the roots as in the branches, this rude pumping process will 
produce a growth of hard tissue proportionate to the stress to be borne. 
These conclusions are supported by the evidence which exceptional cases supply. If 
intermittent mechanical strains thus cause the formation of wood where wood is found, 
then where it is not found, there should be an absence of intermittent mechanical strains. 
There is such an absence. Vascular plants characterized by little or no deposit of dense 
substance are those having vessels so conditioned that no considerable pressures are 
borne by them. The more succulent a petiole or leaf becomes, the more do the effects 
of transverse strains fall on its outer layers of cells. Its mechanical support is chiefly 
derived from the ability of these minute vesicles, full of liquid, to resist bursting and 
tearing under the compressions and tensions they are exposed to. And just as fast as 
this change from a thin leaf or foot-stalk to a thick one entails increasing stress on the 
Superficial tissue, so fast does it diminish the stress on the internally-seated vascular 
tissue. The succulent leaf cannot be swayed about by the wind as much as an ordinary 
leaf; and such small bends as can be given to it and its foot-stalk are prevented from 
affecting in any considerable degree the tubes running through its interior. Hence the 
retentiveness of the vessels in these fleshy leaves, as shown by the small exudation of 
dye; and hence the small thickening of their surrounding prosenchyma by woody de- 
posit. Still more conspicuously is this connexion of facts shown when, from the soft . 
thick leaves before named and such others as those of Echeveria, Rochea, Pereskia, we 
turn to the thick leaves that have strong exo-skeletons. Gasteria serves'as an illus- 
tation. The leathery or horny skin here evidently bears the entire weight of the leaf, 
and is so stiff as to prevent anv oscillation. Here, then, the vessels running inside are 
Protected from all mechanical stress; and accordingly we find that the cells surrounding 
them are not appreciably thickened. j ; i 
_“qually clear, and more striking because more obviously exceptional, is the evidence 
Siven by succulent stems which are leafless. Stapelia Bufonia, having soft procumbent 
VOL. XXV. 3 M 
