120 The Rev. P. Keith on the Internal Structure of Plants. 



TheWood impel feet. — If the root of the Beet {Beta vulgaris) 

 is taken, and cut open by a transverse section, it will be found 

 to exhibit a most beautiful example of the union of the con- 

 centric and divergent la}'ers as inclosed within a bark and 

 inclosing a pith, and remaining at the same time wholly her- 

 baceous. But the mature stem of the full-grown plant ap- 

 proaches in part to the consistence of wood, and points out its 

 affinity to shrubs and trees. 



If the stem of the common Cabbage (Brassica oleracea) is 

 divided transversely, it will exhibit, first, a bark ; then an 

 inclosed cylinder of a firm and compact texture, interspers- 

 ed with a multiplicity of divergent rays, and approaching to 

 the consistence of wood ; and then a large and firm pulp or 

 pith. The same structure pervades the root also, which is 

 furnished with but little pith. In some stages of the plant's 

 growth the concentric layers are not discernible on the trans- 

 verse section; but in its stage of decay the divergent layers 

 disappear, and the woody cylinder separates spontaneously 

 into a number of fine and concentric layers resembling 

 lace. 



The Wood perfect. — If the trunk of a tree or shrub, such as 

 that of the Oak or Elder, is divided by means of a trans- 

 verse section, the parts that are peculiarly characteristic of 

 this division will be rendered visible. The outer portion is 

 the bark. In young subjects it is of a flexible and leathery 

 texture, consisting, first, of an epidermis or external pellicle ; 

 secondly, of a layer of cellular tissue, called the cellular in- 

 tegument ; and thirdly, of a number of thin and concentric 

 layers, designated by the name of the cortical layers, and tra- 

 versed, at least in aged subjects, by a multiplicity of divergent 

 rays. The intermediate portion of the trunk is the wood, 

 constituting the main body of the full-grown plant, and exhi- 

 biting on its horizontal section an indefinite number of con- 

 centric layers, intersected by an indefinite number of diver- 

 gent layers issuing from the centre like the rays of a circle, 

 but often alternating with incomplete rays that do not issue 

 from the centre. The interior portion of the trunk is the pith, 

 a soft and ^spongy substance, of a whitish colour, lodged in 

 the centre of the wood as in a tube. It is always most abund- 

 ant in the young shoot, of which it constitutes the principal 

 mass. But as the wood increases, the pith diminishes, and in 

 old and full-grown trunks it totally disappears. 



The structure of the branches is similar to that of the trunk, 

 as is also the structure of the root with its divisions, at least 

 till you reach the extreme radicles, which present the appear- 

 ence of a cylindrical mass of pulp inclosed in a fine epidermis, 



