46 THE INNER ORGANIZATION OF TREES. 



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 be- 

 tween 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 descending sap or cambium, which spreads itself be- 

 tween the bark and wood ; and it is through the trans- 

 formations 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 parenchymatous 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 parenchyma 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 into a soluble sugaiy 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 

 producing wood, bark forming bark, the cells preserving 

 their original form of parenchyma only in those portions 



