PROCAMBIUM. FASCICULAR CAMBIUM 97 



shaped ends characteristic of prosenchymatous elements. The 

 time of appearance of longitudinal division in the mother-cell of a 

 procambial strand or in other words the proximity of properly dif- 

 ferentiated strands to the growing-point depends as a rule upon the 

 bulk of the strand, small procambial strands often making their 

 appearance much later than large ones. Longitudinal division con- 

 tinues in a procambial strand for some time after it has become 

 differentiated, in the manner described, from the primordial meristem ; 

 secondary transverse divisions are comparatively infrequent, at any 

 rate in typical cases. The adjacent cells of the fundamental meristem, 

 on the other hand, divide equally in all directions, so that the 

 procambial cells soon become much longer than the elements of the 

 surrounding tissues ; the difference is accentuated by the active apical 

 growth exhibited by the individual cells of the procambium. 



Of the several primary meristems it is the procambium which 

 usually remains longest in the meristeniatic condition. Among 

 Gymnosperms and Dicotyledons, in fact, and exceptionally also among 

 Monocotyledons, the majority of the procambial strands that give rise 

 to vascular bundles do not become wholly converted into permanent 

 tissue. A portion of each strand remains permanently in the con- 

 dition of [procambial] primary meristem ; this so-called fascicular or 

 intrafascicular cambium forms a strip which extends tangentially right 

 across the bundle, separating from one another the two principal 

 conducting tissues of the strand, the hadrome and leptome. The 

 cells of the fascicular cambium are at first irregularly disposed, but 

 later arrange themselves in rows which run parallel to the plane 

 of symmetry of the bundle, thus forming a serial cambium. It is 

 in this way that a regular secondary increase of hadrome and leptome 

 is rendered possible. 



The complete discussion of the manner in which these isolated 

 cambial strips subsequently unite to form a closed cambial cylinder, 

 which effects the secondary growth in thickness of the stem [and 

 root], must be postponed until a later occasion (Chap. XIV.). It may 

 be remarked in passing that the vascular bundles of Gymnosperms 

 and Dicotyledons, with their persistent cambial strips, are often 

 described as open bundles, in contrast to the closed bundles devoid of 

 cambium which are characteristic of Monocotyledons and Ferns. 



The procambium primarily represents a homogeneous meristem, 

 although the permanent tissues to which it gives rise may be 

 exceedingly diverse. The uniformity thus attributed to the pro- 

 cambium, however, applies only to its visible histological characteristics. 

 How far this tissue is uniform in respect of its inherent morphogenetic 

 tendencies is quite another question. In other words, it is a matter 



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