332 THE WOODY STEM 



loosely applied not only to the cambial (initial) tayer 

 itself, but also to the undifferentiated tissue mother 

 cells on each side of it. When cell division is rapid 

 relatively to the rate of differentiation, there are several 

 layers of this undifferentiated tissue between the fully 

 formed secondary xylem and phloem (Fig. 54, E). 



The cambial layer in the bundle itself (interfascicular 

 cambium) is really a layer of the desmogen strand which 

 has remained meristematic, and the process of cell 

 division and differentiation of the products (tissue 

 mother cells) into secondary xylem and phloem elements 

 is a direct continuation of the cell division and differentia- 

 tion, which has already taken place in the desmogen 

 strand, producing primary xylem and phloem, and 

 proceeding outwards from the protoxylem and inwards 

 from the protophloem. In the case of the xylem 

 especially the process often goes straight on with no 

 pause, the elements of the secondary xylem continuing 

 in the same radial rows as those of the primary xylem, 

 so that it is impossible to say where the one ends and the 

 other begins. In some cases, however, there is a distinct 

 p^use between the completion of the primary and the 

 beginning of secondary development. 



Each tissue mother cell cut off from a cambial cell 

 on the inside, i.e. towards the primary xylem, may 

 become either {a) a tracheid, (6) a segment of a vessel, 

 (c) a xylem fibre, or {d) by horizontal division a vertical 

 file of parenchyma cells (Fig. 54, 1)). The cells undergo 

 the corresponding modifications of the cell wall, the 

 protoplasm dying in all except the last case. Each 

 tissue mother cell cut off from a cambial cell on the 

 outside, i.e. towards the primary phloem, may become 

 either {a) a segment of a sieve tube with one or more 

 companion cells, (6) a phloem fibre, or (c) by horizontal 



