70 ELEMENTARY BIOLOGY. [viIL 



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 circumference 

 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 btindles, by narrow bands of parenchymatous' 

 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 

 their power of multiplication, and divide by septa parallel 

 with the length of the stem, or root, as well as transverse to it. 

 Thus new cells. are continually being added, on the inner side 



