56 EXOGENOUS STEM — CAMBlUM. 



number of layers whicli intervene between the imperfectly formed 

 circle and the bark. In 1800, a Juniper was cut down in the forest 

 of Fontainbleau, exhibiting near its centre a layer which had been 

 affected by frost, and which was covered by ninety-one woody layers, 

 showing that this had taken place in the winter of 1709. Inscriptions 

 made in the wood become covered, and may be detected in after years 

 when a tree is cut down ; so also wires or nails driven into the wood. 

 As the same development of woody layers takes place in the branches 

 as in the stem of an Exogenous tree, the time when a branch was first 

 given off may be computed by countipg the circles on the stem and 

 branch respectively. If there are fifty circles, for instance, in the trunk, 

 thirty in one branch and ten in another, then the tree must have been 

 twenty years old when it produced the first, and forty when it formed 

 the other. 



In Exogenous stems the pith is not always in the centre. The 

 layers of wood on one side of a tree may be larger than those on the 

 other, in consequence of their fuller exposure to light and air, or the 

 nature of the nourishment conveyed, and thus the pith may become 

 excentric. Zones vary in size in different kinds of trees, and at different 

 periods of a plant's life. Soft wooded trees have usually broad zones, 

 and old trees form smaller zones than young ones. There are certain 

 periods of a plant's life when it seems to grow most vigorously, and to 

 form the largest zones. This is said to occur in the oak between twenty 

 and thirty years of age. 



Cambium. — External to the woody layers, and between them 

 and the bark, there is a layer of mucilaginous semifluid matter, which 

 is particularly copious in spring, and to which the name of Gamhium 

 (cambio, I change, from the alterations that take place in it) has been 

 given (figs. 113, 114 c). In this substance cells are formed, called 

 cambium cells, of a delicate texture, in which the protoplasm and 

 primary utricle are conspicuous. These cells undergo changes, so as 

 to assume an elongated fusiform shape, and ultimately become thick- 

 ' ened pleurenchyma. So long as the primary utricle can be detected 

 they appear to be in an active state, and capable of developing new 

 cells. This cambium layer marks the separation between the wood 

 and the bark, and may be regarded as constituting the active former 

 tive tissue of Dicotyledonous stems. It constitutes the thickening zone, 

 by means of which the stem is enlarged — the cambium cells situated 

 most internally being subservient to the purposes of the wood forma- 

 tion, while the external ones give origin to the new bark. According 

 to Schacht this is the proper nourishing tissue. 



Bark or Cortical (cortex, bark) System lies external to the wood, 

 and, like it, consists of several layers. In the early state it is entirely 

 cellular, and is in every respect similar to the pith ; but as the vascular 

 bundles are developed, the bark and pith are separated, and the former 



