THE FORMATION OF WOOD IN PLANTS. 427 
pointed out. Here, however, the difficulty disappears, since one channel suffices for the 
eurrent alternating upwards and downwards according to the conditions. Moreover 
there has to be found a force producing or facilitating the downward current, capable 
even of drawing sap out of drooping branches; and no such force is forthcoming. The 
hypothesis set forth dispenses with this necessity: under the recurring change of con- 
ditions, the same distention and oscillation which before raised the sap to the places of 
consumption, now bring it down to the places of consumption. A physical process has 
to be pointed out by which the material that forms dense tissue is deposited at the places 
where it is wanted, rather than at other places. This physical process the hypothesis 
indicates. It is requisite to find an explanation of the fact that, when plants ordinarily 
swayed about by the wind are grown indoors, the formation of wood is so much 
diminished that they become abnormally slender. Of this an explanation is supplied. 
Yet a further fact to be interpreted is, that in the same individual plant homologous 
parts, which, according to the type of the plant, should be equally woody, become much 
thicker one than another if subject to greater mechanical stress. And of this too an 
interpretation is similarly afforded. 
Now the sufficiency of the assigned actions to aecount for so many phenomena not 
otherwise explained, would be strong evidence that the rationale is the true one, even 
were it of a purely hypothetical kind. How strong, then, becomes the reason for 
believing it the true one when we remember that the actions alleged demonstrably go on 
in the way asserted. "They are ever operating before our eyes; and that they produce the 
effects in question is a conclusion deducible from mechanical principles, a conclusion 
established by induction, and a conclusion verified by experiment. These three orders 
of proof may be briefly summed up as follows. 
That plants which have to raise themselves above the earth's surface, 
the aetions of the wind, must have a power of developing supporting à 
priori conclusion which may be safely drawn. It is an equally safe à priori conclusion, 
that if the supporting structure, either as a whole or in any of its parts, has to adapt 
itself to the particular strains which the individual plant is subject to by its particular 
circumstances, there must be at work some process by which the strength of the sup- 
porting structure is everywhere brought into equilibrium with the forces it has to bear. 
Though the typical distribution of supporting strueture in each kind of plant may zu 
explained teleologically by those whom teleological explanations satisfy, and though 
otherwise this typical distribution may be ascribed to natural selection acting apart ion 
any directly adaptive process, yet it is manifest that those departures from the typical 
distribution which fit the parts of each plant to their special conditions are explicable 
neither teleologically nor by natural selection. We are, therefore, compelled to admit 
that, if in each plant there goes on a balancing of the p icular strains by the — 
strengths, there must be a physieal or physico-chemical process by which the ojus a 
Ob thie two ‘dre effected: Meanwhile werare equally compelled to admit, à priori, that t 
Mechanical actions to be resisted, themselves affect the internal tissues in such ug - 
t the increase of that dense substance by which they are ee > var 
e that bending the petioles, shoots, and stems must compress BER 
and to withstand 
structure, is an à 
