728 MECHANICAL LAWS OF GROWTH. 



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. But 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 cortex — 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 their position does not allow them to form a layer which increases in size 

 from this cause. 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 cause certain layers, especially the pith, to absorb the 

 water in the tissues more strongly than the others, and consequently to 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 has ceased it is principally 

 the stronger imbibition and swelling of the wood that presses the surrounding layers of 

 tissue outwards and promotes their peripheral growth. 



The intensity of the longitudinal and transverse tensions consequently depends mainly 

 on the addition of water to the turgescent pith and the swelling wood ; any decrease 

 of the turgidity of the pith must cause it to contract, and hence the whole shoot to 

 become shorter and flaccid. This is in complete accord with observation, since withered 

 shoots, i. e. such as have lost water by transpiration, have not only become shorter but 

 also flaccid. Any diminution of the amount of water absorbed by the wood must in the 

 same manner diminish the transverse tension and the diameter of the shoot. A small 

 loss of water in the peripheral tissue when in a state of passive tension does not on 

 the other hand usually cause directly any considerable increase in its tendency to con- 

 tract ; since the increase in its size from turgidity and imbibition are generally much 

 less considerable than in the pith and wood. 



If now there are circumstances which cause a daily periodic change in the quantity 

 of water contained in, the tissues, the result will be also a periodic increase and decrease 

 in the intensity of the longitudinal and transverse tensions. Such a daily periodicity of 

 the tension has been actually discovered by Kraus (/. c. p. 122), who has observed that 

 the longitudinal tension estimated by the difference in length of the pith and the bark, 

 as A\ ell as the transverse tension estimated by the contraction of the bark when detached 



