854 '^ TEXTBOOK OF THEORETICAL BOTANY 



We must, however, adhere for descriptive purposes to some system of 

 terminology, and though the following account does not pretend to finality 

 it appears to agree with the facts so far as they are known. 



In the great majority of apices there is to be found, below the apical 

 meristem, a ring-like zone of meristematic tissue, which is delimited outwards 

 by the vacuolization of the cells of the cortex and inwards by those of the 

 pith. This forms a tubular downward extension of meristem which is called 

 the meristematic ring, or sometimes, but less happily, the residual meri- 

 stem. From this ring the vascular tissues are derived. The relation of the 

 meristem ring to the apical meristem is variable, but it seems to come in 

 most cases from the outer zone of the corpus. 



The next development is that of the procambium. In a few cases the 

 whole ring becomes procambium, and in such cases a continuous ring of 

 vascular tissues is formed from the beginning, as, for example, in Vinca. 

 In the majority of plants, however, the formation of procambium is localized 

 at certain points on the ring, so that a number of separate vascular strands 

 is formed. The cells of procambium are longer and narrower than those 

 of the meristem and they become increasingly vacuolated. They are polygonal 

 in transverse section and arranged at random. Cell divisions are mostly 

 longitudinal, but they are not in a definite order, and the vascular elements 

 derived from them are therefore not in an orderly arrangement either. The 

 procambium is the same tissue as that called desmogen by some authors 

 (Fig. 842). 



Procambium becomes true cambium when its successive divisions become 

 predominantly tangential, so that its cells and those of the derived tissues 

 are arranged in radial rows. The cambial cells also become characteristically 

 flattened as seen in transverse section. This is held to mark the beginning of 

 the phase known as secondary thickening. When it happens that there is 

 no procambium and orderly tangential division begins in the meristem ring, 

 then obviously there is no distinction between primary and secondary 

 structures. The change from one to the other must in any case be gradual 

 and it is impossible to draw a clear line between them. It may be marked, 

 even in the permanent tissues, by no more than some alteration in the 

 character of the xylem elements, for example, by the exclusive formation of 

 pitted vessels and the increase of fibre cells. Even when cambium has been 

 established, the length of its cells and the length of the corresponding tracheids 

 formed increase steadily for some years. The terms " primary " and 

 " secondary " in plant anatomy imply therefore no more than a succession 

 in time, not an opposition of character. 



Where the development of procambium and consequently of vascular 

 tissue is limited to certain points on the meristematic ring, the separate 

 groups of xylem and phloem thus produced are called vascular bundles. 

 When they begin to form they are still separated by unchanged tracts of the 

 meristem ring, for which there are two alternative possibilities. They may 

 become parenchyma, forming the primary medullary rays, or they also 

 may develop, somewhat more slowly, into procambium and eventually 



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