12 BOTANY. 



processes called medidlnry rays. At first the pith is large, and occupies a 

 large proportion of the plant ; the medullary rays, also, are of considerable 

 thickness ; subsequently, by the inci-ease of the old Avedges and the 

 development of new ones between them, the medullary rays become more 

 restricted. Such is the structure of a young shoot during the first year. 

 At the end of the second year, the shoot is found to have increased in 

 diameter by the formation of a zone of vessels consisting of porous and 

 woody tissue, and a zone of fibrous bark, the medullary rays being continued 

 from within outwards, the immber of such zones increasing year after 

 year. 



Taking up the components of stem in proper succession, we begin with 

 a more particular examination of the pitli. This, the central portion, 

 consists of cellular tissue, the cells, often hexagonal, diminishing towards 

 the circumference. Pith is at first of a greenish color, and full of fluid ; 

 subscfiucntly this disappears, leaving a light colored, spongy, dry mass. 

 Sometimes, in drying, it separates into regular cavities, as in the Walnut 

 and Jessamine ; in this case it is said to be discoid or discifnnn. More 

 fre(iuently the cavities, when they exist at all, are of irregular shape. 

 Occasionally there are vessels in the pith ; sometimes, also, regular deposits. 

 The elder exhibits an al)undant pith ; rice paper consists of sections of pith, 

 the exact origin of which is, however, still undecided ; some ascribe it to a 

 species of ^schymomene. The object of the pith is to furnish nourishment 

 to the young buds, for which purpose it is often filled with dextrine or 

 starch, convertible into sugar by the process of vegetation. When the woody 

 circle of the first year is complete, the pith remains stationary as to size ever 

 afterwards. 



The medullary sheath consists of fibro-vascular or spiral vessels imme- 

 diately including the pith, projections of which pass through this sheath into 

 the medullary rays. A few woody fibres are usually intermingled with the 

 spiral vessels. This sheath is in direct communication with the leaf buds 

 and the veins of the leaves, and carries up oxygen liberated by the decompo- 

 sition of carbonic acid or of water, conducting it into the leaves. 



Woody Layers. During the first year, the vascular cylinder consists of 

 an internal layer of spiral vessels forming the medullary sheath, and 

 external bundles of porous and ligneous vessels. Subsequently, the layer of 

 spiral vessels is not repeated, but concentric zones of porous vessels and of 

 pleurencliyma are formed, constituting, in the tree, the woody circles. 

 Exogenous plants are sometimes termed cydo^ens, from their exhibiting 

 these concentric circles. A transverse section of a branch or trunk of a 

 tree usually shows these concentric circles very clearly, each one of which 

 is generally supposed to represent the growth of a year. The circle of large 

 pores usually seen to separate contiguous layers, is composed of the mouths 

 of porous vessels. The distinctness, as well as the size of these circles, varies 

 in different plants, and even in different parts of the same section. Neither 

 is the number of rings in a cross-section to be taken as an indication of the 

 true age of a tree, since there is good reason for supposing that two and 

 even more rings may be formed in a single year, Avhile one ring may occupy 

 12 



