THE INNER ORGANIZATION OF TREES. 45 



it abounds in nutritive matter, which serves to nourish the 

 young buds on its surface, and is often of a green color, more 

 or less intense. But when the buds develope into branches, 

 supporting leaves, flowers, and other appendages, the nutri- 

 tive liquids accumulated in the pith are absorbed, the parti- 

 cles of green matter disappear ; and when the vegetation 

 commenced in Spring is arrested in Autumn, the cells of the 

 pith are dry, colorless and empty: they are then, in fact, 

 dead cells. 



The parenchyma cells of the medullary rays are of a quad- 

 rilateral form, and develope in horizontal radiating lines 

 from the pith to the bark. These lines are easily distin- 

 guished on the cross-section of the stem, when the wood is 

 compact and not too deeply colored : the oak, for example. 

 Their looser structure and lighter color renders them in 

 such circumstances more visible. The medullary rays devel- 

 ope vertically as well as horizontally, and partition off the 

 wood into a number of wedges, in the form of elongated 

 triangles, of which the point that is a little obtuse corres- 

 ponds to the medullary canal. 



The medullary rays are of great service to the old wood. 

 They maintain an exchange of sap between the cells of the 

 pith, wood, and bark; and when the pith cells are quite dead, 

 they unite the older annual layers of wood with the younger 

 and with the bark, and thus continue the communication. 

 They therefore survive the death of the pith cells, and 

 even of the wood-cells, in the midst of which they radiate. 

 The medullary rays of a five or six-years' old wood-ring, are 

 still vitally active cells, filled with sap. 



We have now given the anatomical and physiological 

 peculiarities of the different species of cells which, united, 

 form the tissue or substance of the stem of a beech-tree. 

 We proceed to investigate the physiological phenomena of 

 these cells, as combined together into a continuous tissue, 

 and thus give the reader as clear and philosophical an idea 

 as possible of the growth of the tree. 



The remarkable disposition of the substance of the bark 

 and wood in circular strata or layers, results from the annual 



