■t!S 



THK PLANT CKLL. 



I. The Cambium (found as a distinct layer of meristein in the 

 stem and root of Diootykdons and Coniferae). — Before passing on 

 to the detailed description of the cambium, it is necessary to 

 examine brieHy the arrangement of the tissues in the vascukr 

 region of a dicotyledonous stem, together with the early origin 

 of the cambial layer, and its subsequent history. 



The vascular region proper is that part which lies internal to 

 the endodermis or starch-sheath, a ring of cells which is found 

 immediately internal to the cortex in dicotyledonous or coniferous 



ni^h. 



JCMc 



erid. 



Fig. 31. — A TiiANSVERSE Skction across an OLDEii Stem. — e, Epidermis; 

 a, cork-layer ; c, cortex ; end, endodermis ; re, cambium ring (the line 

 points to an interfascicular portion, ifc) ; ph, phloem ring ; p.ph, rem- 

 nants of the original protophloem ; I, ?, 1, 2, etc., the ring of xylem, 

 made up of xylem elements derived from the fascicular and inter- 

 fascicular cambium respectively; p-xy. protoxylem ; m, medulla or 

 pith. 



stems ; all the tissues internal to the endodermis are included ia 

 the term " central cylinder,"* and comprise, from without inwards, 

 the bast and phloem, the cambium, the wood, and, in the centre, 

 the pith or medulla (see Figs. 31, 32, 33, and 34). All the tissues, 

 however, contained in this central cylinder are not, in the true 



* A central cylinder or "plerome" is also to be found in the young 

 monocotyledonous stem, but the primary cortex appears to merge into it, 

 there being no endodermis proper. 



