74 THE MECHANICAL FUNCTIONS. 



soon as it becomes visible, and its structure can be distin- 

 guished, it is found to contain within itself the parts which 

 are to arise from it, in miniature, and folded up in the small- 

 est possible compass. The portion destined to form the 

 stem is gradually expanded both in breadth and height, but 

 principally the latter; so that it rises as it grows, during a 

 certain period, until the fibres acquire the solidity and 

 strength necessary not only for their own support, but also 

 for sustaining the parts which are to be farther added. In 

 trees this process generally occupies one whole season; 

 during which the growth of the first layer of wood, with its 

 central pith, and its covering of a layer of bark, is free and 

 unrestrained. On the second year, a fresh impulse being 

 given to vegetation, a new growth commences from the up- 

 per end of the original stem, as if it were the development 

 of a new bud: and at the sam^e time a layer of cellular tis- 

 sue is formed by the deposition of new materials on the 

 outside of the former wood, and between it and the bark. 

 This is followed by a second layer of wood, enveloping the 

 new layer of cellular tissue. 



The effect of this new growth is to compress the layer of 

 wood which had been formed during the first year, and to 

 impede its farther extension in breadth. But as its fibres, 

 consisting of vessels and cells, are not yet consolidated, and 

 admit of still greater expansion as long as they are supplied 

 with nourishment, their growth, which is restrained late- 

 rally, is now directed upwards, and there is no farther en- 

 largement of their diameter. From the same cause the pith 

 cannot increase in size; and is even found to diminish by 

 the pressure of the surrounding wood. Thus, the vertical 

 elongation of the entire stem continues during the whole of 

 the second year, and the trunk becomes sufliciently strength- 

 ened by the addition of the second layer on its outside to 

 bear this increase of its height. 



While this process is going on in the w^ood, correspond- 

 ing changes take place in the bark, and a new layer is add- 

 ed on its inner surface, or that which is contiguous to the 

 wood. This layer constitutes the liber. All these new de- 

 positions must of course tend to stretch the outer portions 



