548 



red which this mordant produces when directly mixed with the log- 

 wood decoction. Subsequently, when one of the cotyledons was cut 

 open by Prof. Oliver, to whom, in company with Dr. Hooker, I 

 showed the specimen, we found that the whole of its vascular system 

 was filled with the decoction, which everywhere gave the characteristic 

 reaction. And it became manifest that the liquid absorbed through 

 the rootlets, in the central vessels of which it was similarly traceable, 

 had part of it passed directly up the vessels of the axis, while part of 

 it had passed through other vessels into the cotyledon, out of which, 

 no doubt, the liquid ordinarily so carried returns charged with a 

 supply of the stored nutriment. I have since obtained a verification 

 by varying the method. Digging up some young plants (Marigolds 

 happened to afford the best choice) with large masses of soil round 

 them, placing them in water, so as gradually to detach the soil with- 

 out injuring the rootlets, planting them afresh in a flower-pot full of 

 washed sand, and then, after a few days, watering them with a log- 

 wood decoction, I found, as before, that in less than twenty-four 

 hours the colouring-matter had run up into the vessels of the leaves. 

 Though the reaction produced by the mordant was not so strong as 

 before, it was marked enough to be quite unquestionable. 



As these experiments were so conducted that there was no access 

 to the vessels except through the natural channels, and as the vital 

 actions of the plants were so little interfered with that at the end of 

 twenty-four hours they showed no traces of disturbance, I think the 

 results must be held conclusive. 



Taking it, then, as a fact that in plants possessing them the vessels 

 and ducts are the channels through which sap is distributed, we come 

 now to the further question — What determines the varying permea- 

 bility of the walls of the vessels and ducts, and the consequent vary- 

 ing formation of wood ? To this question I believe the true reply is — 

 The exposure of the parts to intermittent mechanical strains, actual 

 or potential, or both. By actual strains I of course mean those 

 which the plant experiences in the course of its individual life. By 

 potential strains I mean those which the form, attitude, and circum- 

 stances common to its kind involve, and which its inherited structure 

 is adapted to meet. In plants with stems, petioles, and leaves, 

 having tolerably constant attitudes, the increasing porosity of the 

 tubes and consequent deposit of dense tissue takes place in anticipa- 

 tion of the strains to which the parts of the individual are liable, but 

 takes place at parts which have been habitually subject to such 

 strains hi ancestral individuals. But though in such plants the 

 tendency to repeat that distribution of dense tissue caused by 

 mechanical actions on past generations, goes on irrespective of the 

 mechanical actions to which the developing individual is subject, 

 these direct actions, while they greatly aid the assumption of the 

 typical structure, are the sole causes of those deviations in the rela- 



