EXOGENOUS STEM. BARK. 43 



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. 



80. 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 more full 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. 



81. 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 Cambium 

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

 given (figs. 96, 97 c). In this are afterwards formed cells, called cam- 

 bium 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 thickened pleuren- 

 chyma. 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. 



82. 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 

 gradually becomes altered by the formation of secondary deposits. 

 The bark consists of a cellular and vascular system. In this respect 

 it resembles the wood, but the position and relative proportion of these 

 two systems is reversed. In the bark the cellular system is external, 

 and is much developed; while the vascular is internal, and occupies 

 comparatively a small space. The cellular portion of the bark con- 

 sists of an external layer, or Epiphlceum (Ivi, upon, on the outside, and 

 <pWof, bark), and the cellular envelope, or Mesophloeum (ftioo;, middle); 

 while the vascular system forms the internal portion called Liber, or 

 Endophlceum (iv^ov, within). 



83. The inner bark, or endophlceum (fig. 98 /c), is composed of elon- 

 gated pleurenchyma mixed with laticiferous vessels and some cellular 

 tissue. It is separated from the wood by the cambium layer. The pleu- 

 renchymatous tubes are thickened by concentric deposits in their 

 interior, and thus they acquire a great degree of tenacity. The liber 

 of the Lime tree and of Lepurandra saccidora (a species of Antiaris?), are 

 used to form mats, cordage, and sacks; and the toughness of the fibres 



