MECHANICS OF GROWTH. 
the incision was made. But in all cases (the trees already named) the wood outside 
this layer of autumnal wood — and therefore all that was formed after the decrease of 
pressure — consisted of fibres which were not at all flattened radially, but had the same 
diameter, or even one somewhat greater, than those in the middle of the normal 
annual ring ; it contained also as many vessels, or even more, than the normal wood. 
At the time therefore when autumnal wood is being formed in the normal parts of the 
branches, a woody tissue is produced, if the pressure is artificially diminished, agreeing 
in its structure with the ordinary wood formed in the middle part of the annual 
ring. For the normal production of autumnal wood it seems therefore necessary for 
the bark and the bast to exercise a considerably greater pressure on the cämbium and 
the young wood. 
These results explain the older experiments of Knight in 1801. He fastened young 
apple-trees with a stem of about one inch diameter so that the lower part, about three 
feet long, was immoveable, while the upper part with the foliage could bend under 
the pressure of the wind. During the period of vegetation the upper moveable part of 
the stem increased considerably in diameter, the lower fixed part only slightly. This 
is easily explained if we bear in mind that the swaying of the upper parts of the stem 
in different directions by the wind must always stretch the bark on the convex side, 
and therefore eventually relax it; it must thus become looser, and therefore the 
pressure of the bark at these points is always somewhat less than at the lower and 
immoveable parts of the tree. This explanation is completely confirmed by the fact 
that in one of the trees which could be swayed by the wind only in a northerly and 
southerly direction, the diameter of the stem increased so much in this direction as 
to bear the proportion of 13 to 11 as compared with the diameter in the easterly 
and westerly direction. It is obvious that this explanation is much more probable 
than that given by Knight himself, who thought the movement of the sap in the wood 
was promoted by the swaying of the stem caused by the wind. 
The great assistance to the increase in diameter of trees afforded by the diminution 
of the pressure of the bark on the cambium has been long employed in horticulture. 
The bark of young trees is split from above downwards in summer ; cushions of wood 
are formed at the edges of the incisions, which soon close up the wounds. The use 
of this process is that from the more rapid increase of the wood in thickness, the 
conduction of water to the leaves becomes more copious and the loss by transpiration 
is more easily replaced. The development of the buds and hence the formation of 
the organs of assimilation will be promoted by the increase of turgidity in the young 
branches. 
2. If the roots of a plant which is transpiring through its leaves can obtain but a small 
quantity of water from the soil, the turgidity of the tissues in the growing organs will 
be less than it would be if the roots were surrounded by more moist earth. The 
immediate result of this will be, that the cells, and therefore also the organs which 
consist of them (leaves, internodes), will grow more rapidly when the supply of water 
is increased; in this way the assimilating leaf-surface will be increased, and, as a 
secondary result, there will be increased assimilation. This, in conjunction with an 
adequate supply of water, will produce a more vigorous growth of the whole plant, 
which will thus attain a more considerable size and a greater dry-weight than a plant 
which cannot obtain a sufficient supply of water. These far-reaching effects of the 
turgidity produced by an abundant supply of water are very prominent in a series 
of experiments made by Sorauer (Bot. Zeitg. 1873) upon Barley-plants. These plants 
were grown so that all the conditions, except the amount of water in the soil, were 
the same in each case. In one case the amount of water present in the soil was 10 per 
cent., in others 20, 40, 60 per cent, of the amount requisite for complete saturation. 
It appeared the more moist the soil the more fully were the leaves developed, that 
is, the greater the turgidity of the tissues. With the increase of breadth of the leaves, 
the number of the fibro-vascular bundles in them increased; and not only was the 
