35.] CHAPTER II. THE TISSUES. 201 



centre to the periphery of the member (Fig. 149) ; their more or 

 less thickened walls are lignified (p. 132), and they have proto- 

 plasmic contents. Occasionally, however, some of the cells of a 

 ray lose their protoplasmic contents and constitute tracheids (e.g. 

 Abietineoe, Fig. 149 q q) in some few cases the ray consists of 

 long fibrous cells, in place of parenchyma (e.g. shrubby Begonias). 

 The medullary ray is, then, a strand of cells passing radially 

 among the longitudinally arranged tissues of the wood and of the 

 bast (Fig. 149). Its size varies, even in the same member, both 

 as regards its vertical (height) and its lateral (breadth) dimen- 

 sions. With regard to the former, the ray may consist of only a 

 single row of cells (as in Abietinese, Quercus, Fagus) ; the limits 

 may be generally stated at 112 rows of cells, though in some 

 cases they are considerably larger than this when they include 

 resin-ducts (e.g. Abietinese) or other forms of secretory tissue. In 

 any case, the secondary medullary 

 rays, unlike the primary, do not ex- 

 tend throughout the whole length of 

 an internode. The breadth of the 

 secondary medullary rays is never 

 nearly so great as their height : as 

 seen in tangential longitudinal sec- 

 tion, they are narrow above and below Fm - 162.- Diagrammatic repre- 



_ , . . ., .,-,, ... , sentation of the course of the 



and broader in the middle ; it is only me duiinry rays in a segment cut 

 in the middle that they ever consist oufc of fche wood of a tree-trunk. 



,. . , , . . Q Horizontal surface ; E Radial 



oi more than one row of cells in eurface . T Tangential (external) 

 breadth, the upper and lower margins surface of the wood ; the shaded 

 consisting of- a single row only. With portions m are the medullavy ray8 ' 

 regard to their radial extent, it is only the primary medullary 

 rays which extend from pith to pericycle ; the subsequently formed 

 rays (secondary, tertiary, etc.) extend between the wood and the 

 bast of the year in which they were formed. 



As instances of especially large secondary medullary rays should 

 be mentioned those formed in roots (see Fig. 146, p. 194) where the 

 cambium forms only conjunctive tissue opposite the prima.ry 

 xylem-bundles. 



In some few stems the formation of secondary conjunctive tissue 

 is especially connected with the primary medullary rays (e.g. 

 Cucurbitaceae, Menispermacese, woody Piperacese, Aristolochia, 

 Casuarina, Atragene, Begonia, Berberis, etc., see p. 193). In these 

 plants the formation of secondary vascular tissue is confined to the 



