&2 ON VEGETABLE PHYSIOLOGY. 



vey it into the cells of the latter ; in which, as 

 we have stated, the nutritive and other juices, 

 necessary to the growth and preservation of 

 the plant, are elaborated and deposited in their 

 respective receptacles, but whether by glandular 

 strainers or by other agents, experience has not 

 yet decided. We have here another analogy that 

 exists between animal and vegetable nutrition, 

 as far as the effect and object be concerned ; 

 though the instruments employed, and the secre- 

 tions produced, will not admit of a comparison. 



Immediately under the cortical vessels, we find 

 the ivood, composed of an external vascular 

 substance, which, from being white, has been 

 termed the alburnum, and in old trees, of an in- 

 ternal unorganized part denominated the heart 

 or dead wood; the whole surrounding a tube in 

 which is situated the pith ; the wood representing 

 to the eye externally, an arrangement of concen- 

 tric and divergent layers. The concentric or 

 circular lines shew the layer of new wood which 

 has been formed by the cortical vessels each 

 year; and from the number of these, the age of 

 the tree is usually ascertained. The divergent 

 lines, which from their white appearance, have 

 been denominated the silver grain, do not admit 

 of so easy an explanation. 



The alburnum, or sap wood, which may be con- 

 sidered the grand vascular organ of the plant, is 

 made up of cells and tubular vessels ; the cells 



