DEVELOPEMENT OF VEGETABLES. 87 



ble 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 neces- 

 sary not only for their own support, but also for 

 sustaining the parts which are to be further 

 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 unre- 

 strained. On the second year, a fresh impulse 

 being given to vegetation, a new growth com- 

 mences 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 cel- 

 lular 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 sup- 

 plied with nourishment, their growth, which is 

 restrained laterally, is now directed upwards, 

 and there is no farther enlargement of their 



