58 SECONDARY INCREASE IN THICKNESS 



formed outside of it, and so on (Fig. 30, C). A peculiar type 

 is found in the climbing Serjanias of the family Sapindaceae 

 where the stem is traversed vertically by several ridges, which 

 in cross section look like lobes, each containing a circle of vascular 

 bundles surrounding a pith; and the center of the stem is occu- 



p 



FIG. 29. Portion of a cross section through the stem of Dracaena marginata. P, 

 parenchyma of cortex. V, meristematic zone of the pericycle by the activity of which the 

 stem increases in diameter, with the addition of new vascular bundles. M, mature vascular 

 bundle. N, nearly mature vascular bundle. O, newly formed procambium strand from 

 which a vascular bundle is to arise. B, beginning of a procambium strand by the division 

 of cells in the meristematic zone. F, parenchyma of the fundamental tissue. (After 

 Haberlandt.) 



pied by a circle of vascular bundles in the usual way. The stem 

 is ridged when first formed from the primordial meristem, and 

 the primary bundles, following the contour of the stem are laid 

 down in the form of a lobed circle, as seen in cross section. 

 When the interfascicular cambium is formed it extends across 

 the base of each lobe, cutting it off from the central or main 

 part; and then it completes the circle of bundles in each lobe and 

 also the central circle, so that each xylem or wood cylinder is 

 entire and surrounded by a phloem cylinder. 



