34 MECHANICAL LAWS OF GROWTH. 



examined {Acer Pseudo-platatius, Sa/ix cinerea, Popuhis albn, Pavia) the autumnal wood 

 was formed at this spot of fibres flattened radially, between which were a smaller 

 number of vessels than in the normal wood ; its composition was therefore the same as 

 that of the normal ' autumnal wood.' The normal autumnal wood of Ailanthus glandu- 

 losa consists almost entirely of wood-parenchyma-cells flattened radially; while the 

 autumnal wood beneath a ligature made in May consists of a thicker layer of flattened^ 

 fibres, between which only a few vessels could be seen. These results show that 

 when the pressure is increased, the formation of the autumnal wood begins at a time 

 when, under normal pressure, a large-celled woody tissue is still being formed. 



' A diminution of pressure is obtained by making radial longitudinal incisions into, 

 the bast-tissue. The strips of bast contract somewhat tangential) y, since their tension 

 ceases. Near the incisions the pressure of the bast upon the wood is entirely removed; 

 but in the middle between two adjacent incisions a considerable pressure always remains. 

 The fresh portions of tissue which are formed next to the w^ounds differ to the greatest 

 extent in their composition from the ordinary structure of the wood. A layer of wood 

 of the ordinary structure is formed, on the other hand, in the portions of the cambium 

 at the greatest distance from the incisions, and afterw^ards also outside these abnormal 

 portions of tissue. But it is only the tissue consisting of wood formed under artificially- 

 diminished pressure that we have at present to consider.' Incisions 4 to 6 -cm. in cir- 

 cumference, and mostly 2 to 3 cm. long, w^ere made in two- to three-year-old branches 

 in the middle of June and the middle of July, and therefore after the formation of 

 the normal autumnal wood had already begun. ' The effect of the decrease of pressure 

 was first of all shown, after the branches had been cut oft' in the middle of August, by 

 an increase in thickness considerably greater at the spots than above or below them. On 

 the transverse sections the thickness of the annual ring was greatest near the incision 

 and decreased gradually from there to the middle points between two incisions. The 

 layer of wood formed after the commencement of the experiment was often more than 

 twice as thick at the former as at the latter spots.' For a more exact investigation only 

 those pieces were used in which a layer of distinctly flattened fibres of autumnal wood 

 had been formed before the incision was made. ' But in all cases (the trees already 

 named) the wood outside this layer of autumnal wood— and therefore all that formed 

 after the decrease of pressure — consists of fibres which are not at all flattened radially, 

 but have the same diameter, or even one somewhat greater, than those in the middle of 

 the normal annual ring ; it contains 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 



