28 STIGMARIA FICOIDES. 



the inner and outer borders of the zone we find a few cells only about g-^o 

 (•00125) of an inch in diameter. The size of these cells, contrasted with the extreme 

 thinness of their walls, probably explains their almost invariable destruction. 



Plate IX, fig. 52, represents the central part, /, of fig. 51 enlarged 440 

 diameters. The xylem of the vascular bundle is seen at/; whilst at/' we observe 

 a vacant cavity which, interpreting this section by others made further away from 

 the base of the rootlet, we may regard as having been occupied by the phloem 

 elements of the bundle, whilst the cells,/", correspond to those indicated by the 

 same symbol and immediately surrounding the bundle in Plate V, fig. 48. 



In most of the transverse sections of these rootlet bundles made like Plate V, 

 figs. 46 and 47, where the latter are passing through the inner and broader 

 portion of the rootlet cushion, the elements constituting the vascular bundle 

 appear to have their morphological arrangements modified by their contact with 

 the tissue surrounding them. But we find that a change takes place as soon as, if 

 not before, the bundle escapes from the apex (Plate X, fig. 44, h") of the cone of the 

 cushion ; sections of the bundle at and beyond this point assume the features that 

 characterise them throughout the entire length of the free part of the rootlet. These 

 features are seen in all the bundles represented on Plate XI ; but before dealing 

 with them some other points demand attention. The moment the bundle escapes 

 from the apex of the cushion cone, within the rootlet, we find it encased within a 

 small cylinder (Plate X, figs. 43 and 44,/') composed of linear rows of small paren- 

 chymatous cells. These cells are usually as broad as long; but sometimes, as in 

 Plate IV, fig. 53, the innermost of them, /', are more elongated. This figure repre- 

 sents a longitudinal section of a portion of a small bundle, /, enlarged 100 diameters. 

 Owing to the absence of all tissue from the rootlet cavity, g', the bundles and 

 their sheaths rarely occupy their normal position in the centre of these rootlets, 

 but are usually more or less excentric. 



Growth of the Rootlet Bundle. 



We have already seen, from such sections as Plate VI, fig. 9, that the deflec- 

 tion of its vessels to form rootlet bundles took place simultaneously with the first 

 appearance of a rudimentary vascular cylinder ; and that as the cylinder increased 

 in diameter by the addition of centrifugal exogenous growths, a corresponding 

 increase took place both in the diameter of the bundle and in the number and size of 

 its component vessels. Transverse sections of free rootlets demonstrate the order of 

 that development. After collecting and carefully weighing all available evidence, 

 I conclude that the number of the rootlets given off from a Stigmarian root was 

 finally determined during the youngest stage of the development of that root ; no 

 addition to that number being made during its subsequent growth. It is at first 



