THE FORMATION OF WOOD IN PLANTS. 417 
It is not the aim of the foregoing reasoning to show that mechanical actions are the 
sole causes of the formation of dense tissue in plants. Dense tissue is in many cases. 
formed where no such causes have come into play—as, for example, in thorns and 
in the shells of nuts. Here the natural selection of variations can alone have ope- 
rated. It is manifest, too, that even those supporting structures the building up 
of which is above ascribed to intermittent strains, may, in the individual plant of 
a species that ordinarily has them, be developed to a great extent when intermit- 
tent strains are prevented. We see this in trees that are artificially supported by 
nailing to walls; and we also see a kindred fact in natural climbers. Though in 
these cases the formation of wood is obviously less than it would be were the stem 
and branches habitually moved about by the wind, it nevertheless goes on. Clearly 
the tendeney of the plant to repeat the structure of its type (in the one case the 
structure of its species, and in the other ease that of the order from which it has 
diverged in becoming a climber) is here almost the sole cause of wood-formation. But 
though in plants so circumstanced intermittent mechanical strains have little or no 
direct share, it may still be true, and I believe is true, that intermittent mechanieal 
strains are the original cause; for, as before hinted, the typical structure which the 
individual thus repeats irrespective of its own conditions, is interpretable as a typical 
structure that is itself the product of these actions and reactions between the plant 
and its environment. Grant the inheritance of functionally produced modifications ; 
grant that natural selection will always cooperate in such way as to favour those 
individuals and families in which functionally produced modifications have progressed 
most advantageously; and it will follow that this mechanically caused formation of 
ie SE 
sure to understand both how such inoseulating channels are initiated, and how the structures of their component cells 
are explicable. What must happen to one of ihese elongated prosenchyma-cells if, in the course of its development, it is 
Subject to intermittent compressions? Its squeezed-out liquid. while partially escaping laterally, will more og 
escape upwards and downwards ; and while repeated lateral escape will tend to form lateral channels een 
With laterally-adjacent cells, repeated longitudinal escape will tend to form channels communicating with neal 
dinally-adjacent cells—so producing continuous though irregular longitudinal canals. Meanwhile each cell into and 
out of which the nutritive liquid is from time to time squeezed through small openings in its walls, cannot thicken 
internally in an even manner: deposition will be interfered with by the passage of the currents through the pores. 
The rush to or from each pore will tend to maintain a funnel-shaped depression in the deposit around ; and the p 
sage from cell to cell will so acquire just that shape which the microscope shows us—two — with teli 
apices meeting at the point where the cell-membranes are in contact. Moreover, as confirming this interpretation, 
it may be remarked that we are thus supplied with a reason for the differences of shape between these pessiqes from 
One pitted cell to another, and the analogous passages that exist between cells otherwise formed and otherwise con- 
itioned. In the cells of the medulla, and others which are but little exposed to compression, the passages are 
severally formed more like a tube with two trumpet-mouths, one in each cell. This is just the Form which might be 
expected where the nutritive fluid passes from cell to cell in moderate currents, and not by the violent rane caused 
by intermittent pressures. Of course it is not meant that in each individual cell these structures are determined by 
mechanical actions. The facts clearly negative any such conclusion, showing ccm hey BB: many Cases do, 
that these structures are assumed in advance of these mechanical actions. The implication is, that such mechanical 
actions initiated modifications that have, with the aid of natural selection, been accumulated from generation to 
generation ; until, in conformity with ordinary embryological laws, the cells of the parts exposed to such actions 
assume these special structures irrespective of the actions—the actions, however still serving to aid and complete the 
assumption of the inheri 1 type. 
