On Medullosa pusilla. 



225 



Thick-walled elements enclose the xylem of the bundle in an interrupted, 

 hippocrepiform sheath, but do not extend round the phloem. 



The parenchyma contains some gum-canals and presents no peculiarities. 



The only differences between the leaf-base of M. -pusilla and that of 

 M. anglica are in the hypoderma. This zone is rather narrow in M. pusilla, 

 its usual thickness being about 400 fj. ; in M. anglica it ranges from 2 to 



Text-fig. B. — Group of vascular bundles from the cortex, probably resulting from the 

 division of a single leaf-trace ; pd. periderm, marking inner limit of cortex ; sc., 

 internal sclerotic band, marking inner limit of leaf-base. Drawn by Mr. G. T. 

 Gwilliam. x about 60. Scott Coll. 2818. 



3 mm., i.e. it is about six times as thick, on the average, while the general 

 dimensions are only about four times as great. Further, in M. pusilla the 

 hypoderma is much simpler ; as a rule, the sclerotic' strands are in a single 

 rank, and they never stand more than two deep (Plate 13, fig. 4). In the 

 leaf-base of M. anglica the strands are three to four deep (Scott, '99, Plate 5, 

 Phot. 3 ; Plate 12, fig. 14). In M. pusilla the principal strands are about 

 twice as deep as they are wide, and slightly wedge-shaped, widening 



