XVI.] THE BEAN PLANT. 463 



of a ring of vascular bundles surrounding the parenchymatous 

 pith which extends to the very centre in the younger parts 

 of the stem, while, in the older parts, the middle 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. 



The arrangement of the vascular bundles in the Bean is 

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

 part of the central cylinder, but besides these there are two 

 bundles, which are situated outside the cylinder. 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 vascular tissue {medulla or pith) 

 to the outer limit of the cylinder. 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 

 cellsj 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 



