STRUCTURE OF PLANTS. 199 



the season, it is a fluid, the consistency of which gra- 

 dually gets more and more firm as the season advances; 

 but it is a sort of pulp all the time, unless in those 

 stems that are annual, and in them it ripens into a tube 

 or boon, which in flax, hemp, nettles, and many other 

 plants, is covered with a fibrous coat, that is spun 

 into yarn, after undergoing the requisite dressing. 

 Even those annual stems are, however, up to a certain 

 period of their growth, soft and pulpy. The young 

 shoots of pine, deprived of the incipient leaves, and 

 the scales formed from the hybernaculum of the bud, 

 make a tolerable sallad. 



This annual production, when in the soft state, gets 

 the name of cambium , because, whether the stem be 

 annual or perennial, it undergoes a change. In the 

 annuals it becomes boon and fibres, boon only, fibres 

 only, or various textures, according to the nature of 

 the plant. In trees it always separates into two sub- 

 stances, wood and bark; the wood toward the pith, 

 and the wood outward toward the epidermis. The 

 whole annual quantity and the particular portion that 

 goes to each of these formations, depends upon the 

 kind of tree, the circumstances in which it is placed, 

 and the season ; but the influence of seasons and cir- 

 cumstances has not been much alluded to, at least 

 the results have not been so noted, as that one is able 

 to say which kind of treatment shall produce a maxi- 

 mum of bark, and what a maximum of wood. But 

 if the bark does not scale off or exfoliate, there are just 

 as many concentric layers or rings of wood in the 

 heart of the tree, and of bark on the surface of it, as 

 it has grown years. Sometimes it is difficult to trace 



