408 BOTANY. 



are developed on the one hand into tracheidos, which com- 

 pose tlie secondary wood, and on the other into parenchyma 

 and fibrous tissue, composing the secondary cortex (,sw and 

 sc. Kg. 301, C). There always remains a layer of meristem 

 tissue between the secondary wood and cortex thus formed, 

 so tliat the next year an additional increase is made again in 

 exactly the same manner. Thus it happens that the new 

 growth takes place between the xylem and phloem portions 

 last formed, and that the corresponding xylem and phloem 

 parts of any year's growth come at last to be separated by 

 the similar parts of all the subsequent years' growths {fb, 

 Fig. 301, 0). 



The trache'ides are much elongated, with somewhat taper- 

 ing ends ; their walls are thickened, and are more or less 

 copiously supplied with bordered pits. (See Fig. 15, p. 25.) 



518. — The fundamental system of tissues in the stem be- 

 comes divided into two portions by the development of the 

 fibro-vascular cylinder described above. The inner portion, 

 the pith, which occupies the axis of the stem, is composed of 

 parenchyma, which soon loses its vitality, and persists as a 

 mass of thin-walled and generally empty cells. The outer 

 portion, the primary cortex, consists of parenchyma, which 

 is usually chlorophyll-bearing, and a greater or less amount 

 of sclerenchyma or collenchyma. There is frequently a con- 

 siderable development of cork in the primary cortex, and 

 not rarely the whole of the primary cortex undergoes a 

 corky degeneration. Between the fibro-vascular bundles 

 there are broader or narrower plates of tissue, composing the 

 so-called medullary rays, which in the young stems are 

 parenchymatous, but in older ones they are sclerenchyma- 

 tous (Fig. 301«, m, m). In that portion of each medullary 

 ray lying between the cambium layers of two contiguous 

 fibro-vascular bundles there is a layer of meristem tissue, the 

 cambium of the medullary rays, or the inter-fascicular cam- 

 bium. As this is continuous with the cambium of the 

 bundles, there is thus formed a cylinder of cambium, sepa- 

 rating not only the fibro-vascular, but also the fundamental 

 portions of the stem, into two parts [B, Fig. 301). By the 

 formation of new cells by fission in the inter-fascicular cam- 



