14 STIGMARIA FICOIDES. 



exactly the undulating course of those upon which it rested. The result was that 

 these lenticular spaces became widened, without interruption, into what I have 

 elsewhere designated primary medullary rays ; and which thus continued to receive 

 peripheral additions to their length so long as the vascular cylinder continued 

 to increase in diameter (Plate VI, fig. 9, b'). 



When tangential sections are made of any portion of the vascular cylinder, these 

 medullary rays are intersected transversely, and always present, in such sections, a 

 vertically elongated lenticular outline (Plate V, figs. 8, b', and 16, b). On making 

 two such sections of the same ray, one near the cortex and another close to the 

 medulla, as in Plate IX, fig. 12, and Plate V, fig. 13, the former being the 

 medullary and the latter the cortical section, it will be seen that the size of the 

 latter greatly exceeds that of the former. In other words, these rays, which 

 in my ' Memoir,' Part II (' Phil. Trans.'), I have designated primary medullary 

 rays, increase in size as they proceed from within outwards. 



The same result is seen in transverse sections of the vascular cylinder (Plate 

 VII, fig. 14, V b'). These rays were normally filled with an outward extension of 

 the delicate medullary parenchyma, but this tissue has often failed to be preserved. 

 In the section, Plate IV, fig. 7, b' b\ we find the cells of this tissue elongated in 

 the direction of the ray, but in such sections as are made at right angles to the long 

 axis of each ray, as in Plate V, fig. 16, 6, the tissue resembles a delicate small-celled 

 parenchyma. Vascular bundles derived from the xylem cylinder (Plate V, fig. 16,/), 

 to be referred to again more fully, are deflected outwards through these rays on 

 their way to the rootlets. 



A transverse section of an entire vascular cylinder (Plate VII, fig. 14, and 

 Plate VIII, fig. 15, b) exhibits its component vessels grouped in wedge-shaped 

 segments of unequal sizes. This segmentation is almost wholly due to the inter- 

 vention of the primary medullary rays. The variable diameters of the wedges 

 depend upon whether the section has crossed individual rays at their broader part, 

 or at their narrower superior or inferior extremities, where they contract to the 

 dimensions of the ordinary, inconspicuous medullary rays. As already stated, the 

 cellular tissue, extended from the medullary parenchyma to fill the primary rays, 

 appears in such sections as Plate V, figs. 13 and 16, as if it also was parenchy- 

 matous, but in sections like Plate IV, fig. 7, b' it assumes a prosenchymatous form. 

 In vertical radial sections it exhibits a more mural aspect. 1 



Besides these large "primary" medullary rays the vertical laminae of the vas- 

 cular cylinder are separated by numerous smaller " secondary " rays (Plate IV, 

 fig. 7, b", and fig. 20, b", b"). In radial longitudinal sections (Plate VI, fig. 9, &'" 

 b'") these secondary rays are arranged in unequal groups often composed of several 

 superimposed rows of cells. In transverse sections of the cylinder they are un- 

 1 See ' Memoir,' Part II, Plate xxx, fig. 43,// (' Phil. Trans.'). 



