ANATOMY AND PHYSIOLOGY OF PLANTS. 233 



of vessels is observed, and these, with the tissue in 

 which they are placed, form the cortical layers of that 

 period. 



Within this cortical layer, a new production of vessels and of 

 cellular tissue takes place, which being annually repeated, consti- 

 tutes the series of layers of which the bark is ultimately composed. 



15. The new annual layer is of a more membranous, 

 juicy, and flexible nature, and is commonly called the 

 liber or inner bark. 



There is every reason to believe, that the liber is the most impor- 

 tant part of the bark. Tliis is evident, not only from its annual 

 re-production, but also from the injury which trees receive, when 

 they hare been deprived of this layer. 



16. Next to the bark is placed the wood, constituted, 

 like the bark, of vessels and cellular tissue. 



Like it, too, it consists in the young plant, and in the annual 

 shoot of the older one, of a single ring of vessels, which immediately 

 surrounds the pith. In the following year, a new ring of vessels is 

 formed around the first, and in every succeeding year this process 

 is repeated, so that the wood consists at last of a series of rings en- 

 closing each other, the number of which denotes the age of the tree. 



17. The outer ring of new formed vessels is more 

 succulent than those of older growth, and is generally of 

 whiter colour, whence it has received the name of sap- 

 wood or alburnum. 



The vessels which are annually formed and constitute the 

 alburnum, are disposed in radii which extend more or less com- 

 pletely from the circumference to the centre. In some trees these 

 vessels are more numerous than in others ; and in the process of 

 vegetation they frequently undergo great alterations in size and ex- 

 x2 



