8 14 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 cambium 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 eff'ects 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 hot only was the 



