60 MORPHOLOGY OF THE ANGIOSPERMS 



cambium, which functions only briefly. In some genera, "secondary" 

 cambia — cambia arising late outside the primary vascular tissue — may 

 build up the vascular cylinder, as in many arborescent monocots and 

 some families of herbaceous and subshrubby dicotyledons, such as the 

 Chenopodiaceae. These accessory cambia may continue to function 

 permanently or may soon be replaced by similar, later-formed meristems 

 that arise successively in outer tissues. Many woody vines — lianas espe- 

 cially — increase in diameter in this way. 



Histologically, the angiosperm cambium is a sheet of initiating cells 

 of two shapes, fusiform and rectangular or rounded. The fusiform cells 

 build the vertical system; the rectangular, the radial systems of the 

 xylem and phloem. The length of the fusiform cambium initials is 

 closely correlated with the length of the cells formed by them; cells 

 derived from long initials generally increase little in length as they 

 mature. Cells derived from short initials also increase little and, in the 

 higher types of vessel element, the "body" of the cell may be even 

 shorter than the initial from which it arose, with a "tail" showing the 

 length of the initial. Fibers may increase greatly in length beyond the 

 length of the mother initials. 



In the early decades of the twentieth century, the possibility of the 

 penetration of the ends of maturing vascular cells between the walls of 

 adjacent cells as they elongate — "intrusive growth," "gliding growth" — 

 received considerable attention. This growth was considered impossible 

 by some anatomists, partly on the basis that protoplasmic connections 

 would be ruptured; but intensive studies of the cambium and of cam- 

 bium-cell length have shown that intrusive growth is a common feature 

 of wood ontogeny. 



The basic pattern of cellular sti'ucture in xylem and phloem is present 

 in the cambium, and change in structure of these tissues is initiated 

 there. Changes in type, number, and arrangement of cells formed as 

 secondary growth continues are based on changes in the cambium 

 initials. New cells are added with increase in area of the cambium. 

 Fusiform initials form additional similar cells by radial division or by 

 a transverse or oblique division followed by elongation. New ray initials 

 are formed by transverse divisions of fusiform initials. The fusiform 

 cells of the more primitive dicotyledons are long and long-tapering, as 

 are the tracheids, scalariform vessels, and sieve elements formed by 

 them. Specialization in the cambium has been a progressive shortening 

 of the fusiform initials and the development of uniformity in ray-initial 

 clusters. The cambium initials of the vesselless taxa are very long and 

 long-tapering; a general shortening of the initials accompanies the 

 presence of vessels. Within an individual plant, as secondary growth 

 continues, the fusiform initials tend to increase slightly in length for 



