THE ROLE OF THE CAMBIUM IN GROWTH 



559 



vascular 

 cylinder 



branch gap 

 branch trace 



leaf gap 

 leaf trace 



process is telescoped in root tips as compared with stem tips, the three stages 

 following each other in a more rapid succession. (3) Lateral organs (leaves, 

 floral parts, and side branches) of stems arise from peripheral meristems; 

 those of roots (branch roots) from deeply buried meristematic regions (usually 

 from the pericycle). (4) Lateral organs as a rule arise only in the meriste- 

 matic region of stem tips, but in roots usually develop well back in the region 

 of cell maturation. As an end result of the growth process in roots not only 

 is the gross morphology of the root determined but its tissues are constructed 

 according to a pattern which is more or less characteristic for each species. 



The Role of the Cambium in Growth. — The cambium, which is re- 

 sponsible for most of the secondary groivth {i.e. formation of secondary 

 tissues) of plants, is a uniseriate layer of cells 

 which is invariably located between the xylcm 

 and phloem. In most plants in which it oc- 

 curs the cambium is present as an almost con- 

 tinuous sheath of cells extending from just 

 back of every root tip to just below every 

 stem apex. This continuous lamina of cam- 

 bium is broken, in most species, only at the 

 so-called leaf gaps and branch gaps (Fig. 

 iig), which occur just above the strands of 

 vascular tissue which lead to the leaves and 

 stems. Usually, however, even such gaps are 

 found onlv in young parts of the axis, as they . ^ig. 119. Diagram of a por- 

 ,,,,.,,, . r ^1 tion of the vascular cjlinder of 



are gradually bridged by extensions ot the \ 



*=•''' a stem illustrating leaf and 



cambium layer as the stem grows older. True branch gaps. Redrawn from 

 cambiums are found only in the dicots and Eames and MacDaniels (1925). 

 gj^mnosperms, although some monocots in- 

 crease in diameter by means of the so-called "secondary cambiums" which will 

 not be considered in this discussion. 



Structurally cambium cells are of two distinct types. The vascular ray 

 initials from which the vascular rays develop are more or less isodiametric. 

 The vertically elongated elements of the xylem and phloem develop from 

 a second and more abundant type of cambium initial. As viewed in cross 

 section the tangential width of cambium cells of this type is usually several 

 times as great as their radial width. The length of such a cambium cell 

 usually exceeds even the greatest of its cross-sectional dimensions by many 

 times (Fig. 120). The cambium initials of a tulip tree, for example, are 

 about 600 ju long, 25 fx. in tangential width, and 8 /a in radial width. Much 

 longer cambium cells have been reported in the stems of some conifers. In the 



