72 ELEMENTARY BIOLOGY. [CHAP. 



depends on the addition of new cells, takes place chiefly, if 

 not exclusively, in this part. 



The apex of the root, on the other hand, gives rise to a 

 root-sheath, as in the Fern. 



The leaves cease to grow by cell multiplication at their 

 apices, when these are once formed, the addition of new 

 cells taking place at their bases. 



The tissues which compose the body of the Bean -plant 

 are similar, in their general characters, to those found in the 

 Fern, but they differ in the manner of their arrangement. 

 The surface is bounded by a layer of epidermic cells, within 

 which, rounded or polygonal cells make up the ground- 

 substance, or parenchyma, of the plant, extending to its 

 very centre in the younger parts of the stem and in the root; 

 while, in the older parts of the stem, the centre is occupied 

 by a more or less considerable cavity, full of air. This 

 cavity results from the central parenchyma becoming torn 

 asunder, after it has ceased to grow, by the enlargement of 

 the peripheral parts of the stem. Nearer to the circumfer- 

 ence than to the centre, lies a ring of woody and vascular 

 tissue, which, in transverse sections, is seen to be broken up 

 into wedge-shaped bundles, by narrow bands of parenchy- 

 matous tissue, which extend from the parenchyma within 

 the circle of woody and vascular tissue (medulla or pith) to 

 that which lies outside it. Moreover, each bundle of woody 

 and vascular tissue is divided into two parts, an outer and 

 an inner, by a thin layer of small and very thin-walled cells, 

 termed the cambium layer. What lies outside this layer 

 belongs to the bark and epidermis; what lies inside it, to 

 the wood and pith. 



The great morphological distinction between the axis of 

 the Bean and that of the Fern lies in the presence of this 

 cambium layer. The cells composing it, in fact, retain 



