THE INNER ORGANIZATION OF TREES. 45 



more or less intense. But when the buds develope into 

 branches, supporting leaves, flowers, and other appen- 

 dages, the nutritive liquids accumulated in the pith are 

 absorbed, the particles 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 

 quadrilateral form, and develope in horizontal radiating 

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

 distinguished 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 develope vertically as well as horizontally, and parti- 

 tion off the wood into a number of wedges, in the form of 

 elongated triangles, of which the point that is a little 

 obtuse corresponds 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 commu- 

 nication . They therefore survive the death of the pith-cells, 

 and even of the wood-cells, in the midst of which they ra^ 

 diate. 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 an- 

 nual formation of a new stratum or bed of wood on the 

 exterior of that already existing there, and of one or more 



