MODIFICATION OF GROWTH fiAUSED BY PRESSURE AND TRACTION. 813 



they dry up and fall oflf, while those which have formed discs increase in thickness 

 and become woody. 



I. The retarding effect on growth of an external pressure on the cells is very evident 

 in the formation of the annual rings in wood. In the earlier editions of this work 

 I called attention to the fact that the larger radial diameter of the wood- cells in the 

 portion of the rings formed in the spring, and their smaller radial diameter in the por- 

 tion formed in the autumn, might possibly depend on a difference in the pressure from 

 the surrounding bark to which the cambium and the wood are subject, this pressure 

 being less, as we have shown, in the spring, and constantly increasing during the 

 summer. This hypothesis has been fully confirmed by H. de Vries's investigations ^ 

 In branches two or three years old he increased the pressure of the bark in the spring 

 by firmly winding string round them at particular places. The experiment showed in 

 all cases, firstly, that the absolute thickness of the annual ring was less beneath the liga- 

 ture than the mean thickness of the same annual ring at some distance above or below 

 that spot. In several instances the difference was so considerable that the spot where 

 the experiment was made appeared of considerably less diameter even to the naked eye, 

 and this effect was increased by the formation of cushions of wood immediately above 

 and below the ligature. Secondly, the absolute thickness of the ' autumnal layer ' of 

 wood (up to the middle of August, when the increase in diameter of the tree on which 

 the observations were made ceased) was always greater, and generally considerably so, 

 at the spot where the experiment was made, than the normal thickness. In the trees 

 examined {Jeer Pseudo-platanus, Salix cinerea, Populus alba, Panjia) the autumnal wood 

 formed at this spot consisted 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-celis flattened radially; while the 

 autumnal wood formed beneath a ligature made in May consisted of a thick layer of 

 flattened fibres, between which 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 tangentially, 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 wounds 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 afterwards also on the outside of the 

 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. The incisions 

 were mostly 2 to 3 cm. long, and were made in the periphery of two- to three-year-old 

 branches at distances of from 4 to 6 cm. 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 off in the middle of August, by a considerably greater increase in thickness 

 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 com- 

 mencement 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 



» H. de Vries, Flora, 1872, No. 16. 



