132 A TEXTBOOK OF BOTANY [Ch. IV, 3 



for cords, and even a lace-like material serviceable for 

 fabrics. 



The cambium of the fibro-vascular bundles lies between 

 xylem and phloem. It consists of several rows of compact, 

 rectangular, thin-walled, elongated, protoplasm-filled cells, 

 having the meristematic aspect which is always associated 

 with growth (compare Fig. 92). In older stems, lines of cells 

 in the tissue between the bundles (the beginning of the medul- 

 lary rays), become converted into cambium, continuous with 

 that in the bundles, and thus the cambium ring is completed 

 around the stem. It then forms a perfectly continuous cylin- 

 drical sheath between wood and bark on trunks, branches, 

 and roots, and it merges imperceptibly into the meristem of 

 the buds and root tips, which are thus brought into a single 

 continuous growth system ; but it does not occur in leaves. 

 Being a growth tissue, and therefore thin-walled, it is easily 

 torn, which explains why bark is so readily removable from 

 wood, especially in spring when the cambium is most active 

 and tender. Indeed, at this season the cambium can be 

 stripped in long gelatinous sheets from the wood of some 

 trees, notably white pines. In its growth it divides contin- 

 uously in its own plane, the cells on its inner face becoming 

 new xylem elements, and those on its outer face new phloem 

 elements, while the intermediate cells remain cambium. 

 In this manner it builds also the medullary rays of both wood 

 and bark. 



Inside the ring of bundles lie the cells of the pith, exhibit- 

 ing the large sizes and rounded forms associated with storage, 

 whether of food or water. Among them appear very clearly 

 the intercellular air-spaces, which can likewise be traced in 

 other parts of the stem, although it has not been possible to 

 show them in our small scale drawing (Fig. 85). In reality 

 they are parts of a continuous intercellular aeration system 

 which extends from the pith along the medullary rays and 

 through the outer tissues to the lenticels and the exterior 

 air. In the pith, as in other parts, can frequently be seen 



