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PHENOMENA DUE TO THE TENSION OF TISSUES. 807 



power as the principal cause of the longitudinal tension of growing internodes and 

 leaf-stalks before they become lignified, it is on the other hand probable that the 

 imbibition and swelling of the cell-walls are the chief cause of the transverse tension, 

 he wood, where the transverse tension chiefly originates, is, when mature, scarcely 

 adapted for any distension by turgidity ; while at all events in cells or vessels with 

 bordered pits it is altogether impossible. Closed wood-cells, when turgidity is 

 possible in them, cannot however expand greatly; since their own wall and the 

 woody substance which surround them are far too inextensible to stretch to any 

 considerable extent under the influence of hydrostatic pressure. It has, on the other 

 hand, been already shown (Sect. 13) what considerable alterations of dimension the 

 wood experiences especially in the peripheral and radial direction simply by imbi- 

 bition \ Every layer of wood freshly formed on the inside of the cambium-ring has 

 a tendency to grow wider in the peripheral direction, as long as the supply of water 

 is sufficient to cause a decided svvelling of the cell-walls. The cambial tissue is by 

 this means stretched tangentially, and the enlargement of its cells thus caused is 

 increased by turgidity; and from the thinness of their walls it may be assumed 

 that it is their turgidity that protects them from becoming destroyed by compression 

 between the wood and the bark. The elements of the secondary phloem — the bast- 

 cells and the phloem-parenchyma — can scarcely experience any great change of 

 dimensions owing to the swelling of their cell-walls ; the former are indeed thick- 

 walled, but they are not so arranged as to form a layer which increases in size from 

 this cause. The cells of the latter have such thin walls that their swelling produces 

 but little expansion, and experience teaches that they do not increase much in size 

 in consequence of turgidity. Finally, the periderm and the bark dry up and contract, 

 if not to any great extent, yet with considerable force. 



The experience of every year shows that the fissures in the bark — especially 

 of thick trunks at the end of winter in February and March — become deeper and 

 wider, evidendy in consequence of the great swelling of the wood, which at this 

 time contains the greatest quantity of water; while the bark had time to dry up 

 and contract during the dry weather in winter. If the fissures increase in width 

 by the strong tension thus produced — which can be easily seen when fresh — the 

 damp weather in spring causes the bark to swell ; the tension between it and the 

 wood becomes much less, and the production of wood now begins afresh in the 

 cambium. While the wood is becoming thicker during the summer, the bark dries 

 up and shrinks, and the tension between the outside and inside again increases, 

 to cease once more in the following spring. Not only does an annual period of 

 transverse tension thus arise, but this is also the cause, as we shall see presently, 

 of the difference between the spring and autumn layers of wood. 



The statements made in this section may be briefly summed up as follows; — The 

 tissues, at first homogeneous, become first of all differentiated in such a manner that 

 chemico-physical differences are set up, in consequence of which certain layers, espe- 

 cially the pith, absorb the water in the tissues more strongly than the others, and 

 consequently grow more rapidly ; and the layers which are less turgid and grow more 

 slowly are exposed to a passive traction which promotes their growth. After growth 



* [Nevertheless the amount of water of imbibition which a single lignified cell-wall can take up 

 is small. (See Sachs, Ueb. d. Porositat des Holzes, Arb. d. bot. Inst, in Wurzburg, II. 2, 1879.)] 



