ROOTLET BUNDLES. 23 



to the further development of that bundle. But as this outward extension of the 

 vascular cylinder progresses, more laminse than the primary one from which the 

 bundle originated, take part in that extension; hence, as it grows, the bundle 

 increases in diameter as well as in depth. This synchronous development, both by 

 the lateral and superior apposition of new vessels, explains the wedge-shaped con- 

 tour which each of these bundles presents, when the deflected vessels are inter- 

 sected transversely, as in the tangential sections of the vascular cylinder shown in 

 Plate V, fig. 8, Plate IX, fig. 12, and Plate V, figs. 13, 16. In consequence of this 

 mode of development the smaller and oldest vessels of the bundle, forming the 

 apex of the wedge-shaped transverse section, invariably point downwards, or are 

 acropetal when considered in their relations to the root to which the rootlets belong. 



Transverse sections of the xylem cylinder, like Plate VII, fig. 14, show these 

 bundles passing outwards as at / and /'. These sections also demonstrate the 

 increase in the lateral diameter of the bundle as it passes outwards. The two 

 directions of increase are well illustrated by comparing two transverse sections of 

 the same bundle drawn to the same scale; the one, Plate IX, fig. 12, made tan- 

 gentially through a vascular cylinder near its medullary surface, and the other, 

 Plate V, fig. 13, made close to the periphery of the cylinder. The primary 

 medullary ray, b, of fig. 12, like the similar one, Plate V, fig. 16, b, is comparatively 

 narrow, and the rootlet bundle, //, passing through it, is also small, both in length 

 and breadth. Plate V, fig. 13, represents a parallel section of the same ray as 

 fig. 12, but made close to the periphery of the cylinder. Both these figures are 

 equally enlarged 30 diameters. The increase in the magnitude alike of the primary 

 ray, b, and of the rootlet bundle, /, in fig. 13, is conspicuous, and corresponds 

 with what we observe in longitudinal sections of the rays and bundles as seen 

 in transverse sections of the vascular cylinder like Plate "VII, fig. 14,//. 



Each bundle of vessels, thus separated from the xylem cylinder to constitute a 

 rootlet bundle, contains secondary medullary rays between its component vascular 

 laminas, both in its vertical portions (Plate V, fig. 16, d), and for a short distance 

 after they become deflected horizontally (fig. 16, d'). In these positions, unlike 

 those of the cylinder itself (Plate VII, fig. 11), they often consist of two or more 

 vertical rows of cells. 



Transverse sections made of these rootlet bundles after their escape from the 

 periphery of the vascular cylinder assume the various forms represented in Plates 

 IV, IX, and XI, figs. 28 to 36. In this portion of their course the bundles, as 

 seen in such sections, exhibit an irregularly triangular or wedge-shaped outline, 

 the broad base of each wedge consisting of the latest additions to the bundle ; 

 and its narrow apex,/, corresponding to the points indicated by the same symbol 

 in figs. 12, 13, and 16. The figs. 28 — 34 are all taken from the periphery of one 

 vascular cylinder, and are enlarged 80 diameters. Figs. 35 and 36, similarly 



