46 THE INNER ORGANIZATION OP TREES. 



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

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

 bark on the inner surface of the bed of bark formed during 

 the previous year. 



If during winter, when vegetation is in a state of repose, 

 we examine the cross-section of a young stem or branch, we 

 shall find it in the following state. The bark and wood are 

 firmly united with each other, and lying directly between 

 them a bed of parenchyma will be seen, deprived of green 

 granules, which is called the cambium layer. This bed has 

 been formed during the preceding summer, from the descend- 

 ing sap or cambium, which spreads itself between the bark 

 and wood ; and it is through the transformations effected in 

 its cells that new rings of wood and layers of bark are 

 annually produced. Parenchyma is the original form of 

 every species of cells. If we examine with the microscope 

 a section of a young leaf, or root, or any other organ of a 

 plant, we shall find that it is composed entirely of paren- 

 chymatous tissue in the first stages of its development; as 

 growth progresses, these cellules are gradually transformed 

 into fibre cells, and vasiform tissue or ducts. In the same 

 manner, the different species of cells, forming the annual 

 growths of wood and bark, are generated from the paren- 

 chyma cells of the cambium layer. 



During winter we perceive no change in the cells of the 

 cambium layer, which are filled with nutritive matter, whilst 

 the cells of the medullary rays contain starch. As soon, 

 however, as spring commences, the starch granules are con- 

 verted i^ite a soluble sugary gum called dextrine, with which 

 the cells of the cambium layer are speedily gorged, so that 

 the bark and wood are now easily separated. It is out of 

 this viscid mucilaginous matter, or cambium, that the new 

 layer of bark and wood is annually produced. For as the 

 weather gets warmer, the vital activity of the cells of the 

 cambium layer becomes fully aroused, and they generate 

 cells of the same nature as those with which they are organi- 

 cally united, out of the cambium or sap with which their cavi- 

 ties are charged, and elongate into fibre and bast cells, wood 



