76 THE MECHANICAL FUNCTIONS. 



unrestrained. On the second year, a fresh impulse 

 being given to vegetation, a new growth commences 

 from the upper end of the original stem, as if it 

 were the developement of a new bud : and at the 

 same time a layer of cellular tissue 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 further 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 

 laterally, is now directed upwards, and there is no 

 further enlargement of their diameter. From the 

 same cause the pith cannot increase in size ; and is 

 even found to diminish by the pressure of the sur- 

 rounding wood. Thus the vertical elongation of 

 the entire stem continues during the whole of the 

 second year, and the trunk becomes sufficiently 

 strengthened 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 wood, cor- 

 responding changes take place in the bark, and a 

 new layer is added on its inner surface, or that 

 which is contiguous to the wood. This layer con- 

 stitutes the liber. All these new depositions must 

 of course tend to stretch the outer portions of the 

 bark, which had been first formed, and which yield 

 to this pressure to a certain extent ; but, becoming 

 themselves consolidated by the effects of the same 



