THE VASCULAR CAMBIUM 27 



cell division respectively in root cap and the body of the 

 root, present an important but entirely obscure morphogenetic 

 l)rttbleni. 



III. The Vascular Cambium.— The term "vascular cambium" 

 is applied to vertical strips or narrow cylinders of enlargino- 

 and dividing cells which are lateral in position and which give 

 rise to the secondary phloem and secondary xylem tissue-systems. 

 The vascular cambium is properly regarded as a " secondary meri- 

 stem" since its activity is responsible for the addition, at some 

 distance from the apex of root or shoot, of new or secondary 

 vascular tissues to the original or "primary" conducting system 

 which in turn had its origin in the provascular meristem or 

 "procambium." In many herbaceous angiosperms, especially 

 many of the monocotyledons, and in most of the lower vascular 

 plants, cambial activity is reduced or absent and the vascular 

 system is therefore largely "primary" in character. But in 

 woody angiosperms and in the gymnosperms, the primary tissues 

 of stem and root are short-lived and become destroyed or buried 

 by the more massive secondary vascular system formed by the 

 cambium. 



The most significant of modern studies on the structure and 

 growth of the cambium have been made by Bailey (1920, 1923, 

 1930), who has studied both fixed as well as living cells in a 

 wide range of gymnosperms and angiosperms. From a morpho- 

 logical standpoint, the cambium may be regarded as a single layer 

 of cells in which tangential (i.e., periclinal) divisions predomi- 

 nate during the propagation of phloem or xylem. Two prin- 

 cipal types of initials occur in the cambium, viz.: (1) the 

 fusiform initial, which as seen in tangential longi-sectional view 

 is prosenchymatous in form and in certain plants, according to 

 Bailey, may attain the enormous length of 5,000//, and (2) the 

 vascidar-ray initial, which is a much smaller cell and is more or 

 less isodiametric in form. The fusiform initials form such ele- 

 ments as tracheids, vessels, fibers, wood-parenchyma, and sieve- 

 tubes, while the ray initials are points of origin and propagation 

 of the radially-disposed phloem and xylem rays (cf. Barghoorn, 

 1940). One of the many interesting features of cambial cells is 

 their highly vacuolate character, which is only evident when liv- 



