THE FORMATION OF WOOD IN PLANTS. 411 
placing them in water, so as gradually to detach the soil without injuring the rootlets, 
planting them afresh in a flower-pot full of washed sand, and then, after a few days, 
watering them with the logwood 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 disturb- 
ance, 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 permeability of the walls of the vessels and ducts, and the con- 
sequent varying 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 cir- 
cumstances 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 anticipation 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 in ancestral individuals. 
But though in such plants the tendeney 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 devia- 
tions in the relative thickenings of parts which distinguish the individual from others of 
its kind. And then, in certain irregularly growing plants, such as Cactuses and Euphor- 
bias, where the strains fall on parts that do not correspond in successive individuals, we 
distinetly trace a direct relation between the degrees of strain and the rates of these 
changes which result in dense tissue. I will not occupy space in detailing the evidence 
of this relation, which is conspicuous in the orders named, but will pass to the question, 
What are the physical processes by which intermittent mechanical strains produce this 
deposit of resistant substance at places where it is needed to meet the strains ? We have 
not to seek far for an answer. If a trunk, a bough, a shoot, or a petiole is bent by à 
Sust of wind, the substance of its convex side is subject to longitudinal tension, the sub- 
Stance of its concave side being at the same time compressed. This is the primary 
mechanical effect. "There is, however, a secondary mechanical effect, which here chiefly 
Concerns us, "That bend by which the tissues of the convex side are stretched, also pro- 
duces lateral compression of them. Buttoning on a tight glove and then closing the 
d, will make this necessity clear: the leather, while it is strained along the backs 
of the fingers, presses with considerable force on the knuckles. It is demonstrable 
