106 ANATOMY OF VEGETABLES. 



of vitality. The heartwood is usually much more du- 

 rable timber than the sapwood. In the arrangement 

 of the fibres of the wood there are two distinct ap- 

 pearances. Besides the layers just described, there 

 are a series of white and shining lamellae, which shoot 

 from the centre towards the circumference, and these 

 constitute what is called the Silver grain PL 3, fig. 

 17, d. 



The appearance of the silver grain is generally ob- 

 vious in trees and shrubs, and particularly so in the 

 Oak. Something analogous to it is discoverable in an- 

 nual herbs. 



The silver grain is elastic and susceptible of change 

 of volume by various temperatures. 



III. Medulla or Pith. 



This is situated in the centre of the trunk and 

 branches of plants* JLn plants of one year's growth it 

 is often very large and full of the juices of the plant ; 

 in older ones, it becomes dry, soft, light, and very com- 

 pressible. In most old trees it is altogether obliterat- 

 ed. It has been thought to be a reservoir of the juices 

 in young plants. The pith is very conspicuous in the 

 Elder, Ash, and Sumach. 



General texture of Plants. 



Mirbel finds, by the help of the highest magnifying 

 powers, that the vegetable body is a continued mass of 

 tubes and cells ; the former extended indefinitely, the 

 latter frequently and regularly interrupted by trans- 

 verse partitions. These partitions being ranged al- 

 ternately in the corresponding cells, and each cell in- 

 creasing somewhat in diameter after its first forma- 

 tion, except where restrained by the transverse parti- 

 tions, seems to account for their hexagonal figure.* 

 See PI. 16, fig, 1 1. a. The membranous sides of all these 

 cells and tubes are very thin, more or less transparent. 



* Jn microscopic figures they are generally drawn like circles intersecting each 

 other, 



