ON VEGETABLE PHYSIOLOGY. 63 



being constantly filled with the rising sap, which, 

 after undergoing some changes, is taken up by 

 the tubes and conveyed to the leaves to their 

 utmost extremity ; and having from them receiv- 

 ed a new modification, the sap is returned by the 

 cortical vessels (as have been previously explain- 

 ed) back to the trunk, and thence to the minutest 

 filaments of the roots. 



The heart wood, which, when present, is seated 

 below the alburnum, seems principally intended, 

 (like the bones of animals,) to give solidity, form, 

 and support to the more solid parts of the plant, 

 and is mostly observable in old trees. 



The pith is a soft and spongy, but often succu- 

 lent substance, occupying the centre of the root, 

 stem and branches, and extending in the direction 

 of their longitudinal axis, in which it is inclosed 

 as in a tube. In its structure it is exactly similiar 

 to that of the cellular tissue of the bark ; being 

 composed of an assemblage of cells containing 

 a watery and colourless fluid. In the first infan- 

 cy of the plant, the pith occupies but a small 

 space ; it afterwards gradually dilates, and in 

 young trees offers a considerable diameter. In 

 those more advanced, pressed and acted upon 

 by the heart wood, it begins to diminish ; and 

 in very old forest trees it disappears altogether. 

 Having ascertained that a portion of it may be 

 abstracted without injury to the plant, naturalists 

 consider the pith only of secondary importance ; 



