GROWTH OF VASCULAR BUNDLES 49 



radial divisions; so that in cross sections the vascular bundle 

 has the form of a wedge with its apex pointing toward the center 

 (Figs. 22, 23 and 24). As this wedge broadens new or secondary 

 medullary rays are from time to time begun by the fascicular 

 cambium (Fig. 24). These rays average less than half a milli- 

 meter in vertical extent, although in a few instances they run 

 100 to 200 millimeters from node to node; and in their tangential 

 diameter they are seldom more than five hundredths of a milli- 

 meter, while radially they keep pace in growth with the phloem 

 and xylem, and so always extend from the place of their origin 

 in the xylem out between the phloem strands. The stimulus 

 to form more medullary rays seems to come from the need of 

 more radial highways as the diameter of stem and root and 

 absorbing surfaces of new roots and food-building tissues of 

 new leaves increase. The details of this will be discussed in 

 Chapters VII and X. The daughter cells of the fascicular 

 cambium, in becoming secondary medullary ray cells, enlarge 

 chiefly in their radial and tangential diameters, becoming 

 elongated radially, as a rule, in the xylem, but often vertically in 

 the phloem. 



While the vascular bundle is thus enlarging and secondary 

 medullary rays are being laid down, the interfascicular cam- 

 bium is adding new cells to the primary medullary rays and 

 thus causing them to keep pace in radial growth with the vas- 

 cular bundles. Not infrequently, however, the interfascicular 

 cambium forms new vascular bundles which, in cross section, 

 appear to cut the primary ray into narrow strips. 

 • In the xylem portion of both primary and secondary medul- 

 lary rays the tangential walls remain relatively thin or become 

 pitted, and the radial walls are thin or pitted where they come 

 into contact with tracheal tubes, tracheids, or xylem paren- 

 chyma; while the transverse walls are apt to be much thickened 

 and lignificd. In the phloem portion of the rays the walls remain 

 thin and unlignified. 



Fig. 24 shows that the cambium adds much more to the 

 xylem than it does to the phloem. The growth of the xylem 

 4 



