ORGANS OF NUTRITION. 83 



takes fifteen men with their arms extended to embrace it, and 

 many other remarkable examples might be given of exogenous 

 trees attaining to an enormous size, which circumstance is of 

 itself, also an evidence of their great age. The necessary limits 

 of this volume will not, however, allow of our dwelhng further 

 upon this subject. 



Cambium or Cambium-layer. (FigsAGQ, c, 169, c, and 171,e.) 

 —On the outside of each annual zone (as we have already seen) 

 a layer of vitally active cells is placed, to Avhich the name of 

 cambium or cambium-layer has been given. It is from this 

 that the new layers of wood and bark are formed, and from 

 the fact also of its being situated on the outside of the vas- 

 cular bundles of which the wood is composed, these owe their 

 indefinite power of increase. The cells of which the cambium 

 layer is composed are of a very delicate nature, and consist of 

 a thin wall of cellulose, within which is situated a primordial 

 utricle, a nucleus, and abundance of protoplasm ; in fact they 

 contain all the substances which are present in young developing 

 cells. This layer is dormant during the winter, at which time 

 the bark is firmly attached to the wood beneath, but it is in full 

 activity in the spring, when it becomes charged with the elabo- 

 rated sap of the plant, or that sap which contains the materials 

 necessary for the development of new stnictures, and then 

 the bark may be separated from the wood beneath. It is proper 

 to notice, however, that there is no real interval at any season 

 between the bark and wood, but the two are always organically 

 connected by the delicate cells forming the cambium-layer, 

 and can only be separated therefore by the rupture of the cells 

 of which that layer is composed. The sap which thus fills 

 the cells of the cambium-layer was originally called cambium, and 

 hence the name of cambium-layer applied to this portion of an 

 exogenous stem. The cells of the cambium-layer as growth 

 commences in the spring, when they are filled with protoplasm 

 and other nitrogenous materials, soon begin to multiply by 

 the ordinary process of cell- division already described. By 

 their multiplication transversely the stem increases in a longi- 

 tudinal direction, and by their multiplication perpendicularly, 

 either radially or tangentially, the stem increases in thick- 

 ness, and thus new layers of wood are foimed on its inside, 

 and new layers of bark on its outside. 



3. Medullary Rays.—SSI'Q have already seen that the stem at 

 its first development consists entirely of parenchyma, but that 

 in a short time fibro-vascular bundles are developed, by which 

 it becomes separated into two portions — an internal or pith, 

 and an external or bark; the two being connected by tissue of 

 the same nature as themselves, to which the nsime medullary 

 rays has been applied. These constitute the silver grain of 

 G 2 



