new layers the tree has crowded off the block, drawing the head of the spike directly through the pine wood;
that is, new material has been thrust in between the wood of the tree on the one hand and the block on the other, until the block has been fairly wedged from its place. Now, it is asserted on excellent authority that the force necessary to accomplish this result amounts to a pressure of about thirty pounds to the square inch; i. e., the forces of growth in a soft maple are capable of exerting in all directions a force of thirty pounds to the square inch. Now, these results may seem somewhat surprising, but our surprise is in no degree lessened when we begin to study the machinery by which this energy is exerted. If we could make a cross-section of one of the trees in question, we should find by far the greater part of the tree in a condition of nearly absolute fixity, incapable of enlargement in any direction. Outside is the bark, likewise largely incapable of exerting force, most of the cells haying long since yielded up their living matter. Only on the line of division separating bark from wood do we find a structure whose cells are capable of life, growth, and multiplication. This structure is so thin that only the finest line would be needed for its delineation, were we to draw the whole section, natural size (Fig. 2).
|Fig. 2. — Cross-Section of an Exogenous Stem.|
Furthermore, this layer is made of cells whose walls are exceedingly delicate and thin. So much more feeble, in this regard, are the cells here than on either side, that this layer, the cambium, is the line of separation when, in the growing season, you easily strip the green bark from the wood. The energy, then, which we have estimated, must finally rest upon these thin-walled, delicate cells. Not only is this true, but we may also easily conclude that all the pressure by which the cleat is wedged from the tree must come from the growth and multiplication of the same diminutive organisms. It is plain here that the force concerned is not capillary, for that is certainly as active in the woody parts of the tree as in the cambium, there producing no expansion whatever. Neither does it seem that the energy expended must be attributed to osmosis, although the cell may be by construc-