CIRCULATION AND FORMATION OF WOOD IN PLANTS. 577 



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 proportion- 

 ate to the stress to be borne. 



These conclusions are supported by the evidence which excep- 

 tional 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 con- 

 ditioned 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 sup- 

 port 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 deposit. Still more con- 

 spicuously is this connexion of facts shown when, from the soft thick 

 leaves before named and such others as those of JEcheveria, Rochea, 

 Pereskia, we turn to the thick leaves that have strong exo-skeletons. 

 Gasteria serves as an illustration. The leathery or horny skin here 

 evidently bears the entire weight of the leaf, and is so stiff as to pre- 

 vent any oscillation. Here, then, the vessels running inside are pro- 

 tected from all mechanical stress ; and accordingly we find that the 

 cells surrounding them are not appreciably thickened. 



Equally clear, and more striking because more obviously excep- 

 tional, is the evidence given by succulent stems which are leafless. 

 Stapelia Bu/onia, having soft procumbent axes not liable to be 

 bent backwards and forwards in any considerable degree by the 

 wind, has, ramifying through its tissue, vessels that allow but an ex- 

 tremely slow escape of dye and have unthickened sheaths. Such of 

 the Euphorbias as have acquired the fleshy character while retaining 

 the arborescentgrowth,like Euphorbia Canaricnsis,tea,ch us the same 

 truth in another way. In them the formation of wood around the 

 83 



