M IXr TK S Tit UC T URK. 



r.i 



Ferns and Club-Mosses, as well as of some monocotyle- 

 dons, however, a different arrangement prevails, tlie 

 xylem occupying the central part of the bundlo, ajid the 

 phloem forming' a circle around it. The former arranjre- 

 ment is described as collateral, wliiio the latter is vou- 

 centric. In many of tlie monocotyledons. ,-.>. well as in 

 the exogens, the bundles are collateral. 



Fig. 282 shows a section of an exogenous stem somewhat 

 older than that shown in Fig. 279. Hero new bundles 

 have been formed between the earlier ones, .so that tlio 

 whole centre of tlie stem, excei)t the pith and the lim-.s 

 radiating from it, is occupied by the wood. This cylinder 

 of wood is now encircled by a ring of cambium, beyond 

 which are the tissues of the phloem. 



The ajipearance presented by the cross-section of an exogenous 

 stem is that of a series of concentric rings, each ring 

 showing the limit of a year's growth, The portions of 

 wood formed late in the summer are more compressed by 

 the outlying tissue than those formed in spring, and 

 hence the outer part of each 3-ear's ring appears denser, 

 and is sharply marked off from the ring of the following 

 year. No growth of the cambium takes place in the 

 winter. The rays which intersect these rings as fine 

 lines consist of portions of the ground or fundamental 

 tissue which have been squeezed into their present form 

 bj' the increasing libro-vascular bundles on each side of 

 them ; thej' are called medullary rays, and, as the stem 

 grows, new ones are formed from the cambium. Only 

 the primary ones, however, extend from the pith to the 

 bark ; those formed later are shorter. 



In roots a special arrangement of the tissues of the bundles 

 prevails, the xj-lem and phloem forming alternate rnyx. 

 This is the radial ari-augement. 



Fundamental or Ground-Tissue. 



The Fundamental or Ground-Tissue comprises all the parts of 

 the plant not already included in the epidermal an<l tibro- 

 vascular systems. The collenchyma found just beneath 

 the epidermis, sclerenchyma occurring in different parts, 

 and laticiferous tissue are constituents of the fiuidameutal 

 system as well as the cork cells already referred to. In 

 the monocotyledons ground-tissue in the form of paren- 

 chyma fills the space between the closed bundles of the 

 stem ; while in many plants in which fibro-vascular 

 bundles are not produced, the ground-tissue constitutes 

 the whole of the interior. 



In exogenous stems the wood developed from the cambium is 

 often different from that of the primary bundle as de- 

 veloi^ed from the procambium. Pines, for example, have 



KiK- s»«- 



