ROOTLETS. 27 



cushion seems to fill a vertically elongated lenticular cavity in the prosenchyma- 

 tous zone, which cavity reminds us of the forms of the lenticular sections of the 

 primary medullary rays as seen in tangential sections of the vascular cylinder. 



Sections made parallel to figs. 46 and 47, but intersecting and successively 

 encroaching upon the basal part of a rootlet, still exhibit the tissue, h, though 

 in circles of decreasing diameter, until we reach the conical apex of the cushion 

 (Plate X, fig. 44, h"), when it disappears. Longitudinal sections of the base of the 

 rootlet reveal the existence of a thin layer of a very peculiar tissue springing from 

 the entire conical surface of the cushion. It is composed of elongated branching 

 tubular cells, having a diameter of from -g-^o ("0012) to Y6oo ('0006) of an inch. A 

 few detached examples of these cells are represented in Plate X, fig. 50. The 

 cells being frequently much disorganised, I was long ignorant of their true 

 arrangement, but since Plate X, fig. 43, was drawn I have discovered them at the 

 angles, i i, of the rootlet cavity, radiating upwards and outwards from the surface of 

 the cushion in parallel lines ; the lowest of these lines reach the cortical layer, g, 

 of the rootlet ; the more central ones, pursuing a parallel upward and outward 

 course, have been merged in the. cells of the zone (Plate IX, fig. 51, <j) of this part 

 of each rootlet yet to be described. 1 



Plate IX, fig. 51, represents a transverse section of the basal portion of a 

 rootlet enlarged 50 diameters. The conical part of the rootlet cushion is inter- 

 sected transversely, forming the dark-coloured central zone, h", enclosing the 

 vascular bundle, /. Externally, we have some of the parenchyma of the root-bark 

 at d, within which is the outer or cortical zone of the true rootlet, g. The close 

 continuity of the cells of these two zones shows that this section has been made 

 just below the line x x, of Plate VI, fig. 45, where the parenchymatous layer of the 

 cortex, d, has been bent back upon itself to form the outermost layer, g, of the 

 rootlet. That this outermost rootlet layer is merely an extension of the outermost 

 layer of the bark is demonstrated by all the sections in which both are preserved 

 together. Between the two zones, Plate IX, fig. 51, g and h", we have the very dis- 

 tinct parenchymatous zone, g', which is not preserved in one section in a thousand, 2 

 even in this basal portion of a rootlet, its place being almost invariably occupied 

 by white infiltrated mineral matter. It is, however, well preserved in this 

 section, as is also the case in a young longitudinal section of the corre- 

 sponding part of a rootlet. (Cabinet, No. 746.) The cells of this tissue chiefly 

 range between yg^o ("00437) and T ^y (= -0025) of an inch in diameter. At 



1 These cells have now been introduced into Plate X, fig. 43, i. 



2 It is a noteworthy fact that tissue occupying this position is always absent from the rootlets of 

 Isoetes lacustris ; but in this plant the rootlet bundle, enclosed in its investing cylinder, becomes finally 

 united by a few cells to the inner side of the cortical wall of the rootlet. This union always takes place 

 on the same side of the Isoetes rootlet, viewed relatively to the position of the deep fissure intersecting 

 the base of the stem of this plant. 



