XVI.] THE BEAN PLANT. 463 



centre in the younger parts of the stem, while, in the older 

 parts, the centre is occupied by a more or less consider- 

 able 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. 



The arrangement of the vascular bundles in the Bean is 

 not quite regular. Most of them, as already stated, form 

 a ring between cortex and pith, but besides these there are 

 two bundles, which are situated outside the ring. Each 

 of these occurs opposite one of the projecting corners of the 

 stem. Each vascular bundle consists of two halves; the 

 xylem or wood, which is turned towards the centre of the 

 stem, and the phloem or bast, which is turned towards its 

 periphery. Bundles with this arrangement of xylem and 

 phloem are termed collateral. The wedge-shaped bundles 

 of the ring are separated from one another by narrow bands 

 of parenchymatous tissue, which extend from the paren- 

 chyma within the circle of woody and vascular tissue 

 (medulla or pith) to that which lies outside it. These are 

 the medullary rays. In each of the bundles the xylem 

 and phloem are separated by a thin layer of small, and very 

 thin-walled cells, termed the cambium layer. In the older 

 parts of the stem this layer extends across the medullary 

 rays between the bundles so as to form a continuous ring 

 all round the stem. The tissues inside this layer are the 

 wood and pith, while those outside it are the bast, cortex, 

 and epidermis. 



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. Thus new cells are 



