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. 
The 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 swelling 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, evidently 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. Porosität des Holzes, Arb. d. bot. Inst, in Würzburg, II. 2, 1879.)] 
