344 TRANSACTIONS AND PIIOCEEDINGS OF THE [Sess. lxiv. 



is extremely rare to find an individual from which they are 

 entirely absent, though in some cases they may be reduced 

 to one or two in an internode. 



The Stem. — In S. Dulcamara the leaves are arranged in 

 a two-fifths spiral, and at each node two vascular bundles 

 unite under the base of the leaf ; of these, one arises from 

 the similar vascular union below the leaf two internodes 

 lower down, and the other from that below the next lower 

 leaf, i.e. the leaf three internodes below the first, conse- 

 quently each internode is traversed by five primary vascular 

 bundles, the position of which is indicated on the surface 

 of the young stem .by a corresponding number of well- 

 marked ridges. 



The primary bundles are, as has been pointed out by 

 De Bary (1), bicollateral in structure, while the internal 

 phloem, itself of considerable thickness, is also accompanied 

 by isolated phloem strands. The bundles are of consider- 

 able width in the tangential direction as compared with 

 their rather small radial diameter, and are split up into a 

 number of narrow wedges, each composed of from two to 

 eight radial rows of xylem elements, by a varying number 

 of medullary rays, which extend from the inner limit of 

 the xylem to the outside of the external phloem, without 

 however traversing that on the inner face of the bundle. 



These rays are identical in structure with those laid 

 down in the secondary wood, through which they also are 

 continued, both being as a rule but one cell wide tan- 

 gentially, and from ten to fifteen high, while in botii the 

 component cells retain their protoplasmic contents and are 

 rectangular in outline, with the radial diameter about 

 one-third of the heiglit, and somewhat greater than the 

 tangential width. 



lender normal conditions the course of sucli a medullary 

 ray through the phloem till it conies in contact with tlie 

 inner wall of the jjericycle may be readily traced, owing to 

 the reguliirly radial arrangement of the, in most cases, 

 single row of cells, whicli, moreovei', differ individually, l)oth 

 as regards sliape and contents, from the elements of the 

 phloem by wliich they are bounded on both sides. 



Its course within the xylem is ecjually well delined, but 

 as the walls are now lignified, thougli thinner than tiiose 



