THE STEiM 99 



and Conifers. The composition of each bundle also differs. It 

 is true in their first stages both are similarly made up of thin- 

 walled cells containing much protoplasm, and are then called 

 procambial strands ; but during their later development they 

 become differentiated in different ways. The differentiation of 

 the procambium cells into wood vessels (xylem) (Fig. 12, A, x), 

 and sieve-tubes or phloem (Fig. 1 2, A, p) begins at the peri- 

 phery of each bundle, and proceeds towards its centre. But 

 in the bundles of Conifers, and of most Dicotyledons, there 

 remains in the centre an undifferentiated zone of the original 

 thin-walled cells with their protoplasmic contents, and these 

 retain and use their power of division, thus increasing in 

 number. This persistent zone of cells, which are able to divide 

 (meristematic cells), is absent from the bundles of Monocoty- 

 ledons, and the several bundles cannot therefore increase in 

 thickness during the succeeding years, as is the case with the 

 vascular bundles of most Dicotyledons. 



If, at the beginning of a new period of growth, long-lived 

 plants require an increase of their conducting tissues, this can 

 only be done in the case of Monocotyledons by the formation 

 of new vascular bundles, while in the case of Dicotyledons the 

 number of conducting elements can be increased within the 

 already existing bundle. 



For the purpose of answering the question placed at the 

 head of this paragraph, we must remember that the stems of 

 our woody plants have a ring of bundles inserted in their 

 ground tissue But as all these bundles are arranged in the 

 same way, so that the xylem or wood is turned towards the 

 centre (Fig. 12, .5, x), while the phloem or bast is turned 

 towards the outside (Fig. 12, £, jp, and h, b, b), the xylems 

 form a cylinder which is only interrupted by the narrow 

 medullary rays. The latter pass through the phloem too, and 

 their cells have the faculty of fitting themselves to the tissue 

 through which they have to pass. As long as the medullary 

 ray runs through the wood, its cells are lignified, but in the 

 bast the cells partake of the softer nature of this tissue. 

 Hence the two portions of the ray may be termed the inter- 

 faseieular xylem (Fig. 12, 0, if h) a,nd intepfaseieular phloem, 

 respectively (Fig. 12, C, ifp). By this adaptation of the 



