222 



Dr. D. H. Scott. 



The best leaf-base measures about 13 mm. in the tangential and 8 mm. in 

 the radial direction. 



The tri-stelar vascular system of the stem (fig. 3) is enclosed in a definite 

 but irregular ring of dark tissue, which, judging from the best-preserved 

 portion, is evidently an internal periderm (cf. text-fig. B). The approximate 

 dimensions of the region enclosed by the periderm are 7 x 5*5 mm. The 

 general structure is clearly the same as that of M. anglica, in which three 

 leaf-bases appear in the transverse section, and the vascular system is also 

 normally tri-stelar (Scott, '99, text-fig., p. 126, Plate 5, Phot. 1). In the 

 specimen of M. anglica referred to, the dimensions in its present condition are 

 10'5 x 3 - 7 cm. The natural diameter would no doubt have been a little over 

 7 cm., and the other specimens investigated do not differ greatly in size. In 

 M. pusilla the natural diameter cannot be directly measured, as two of the 

 leaf-bases are crushed and incomplete. Judging from the radius of the best- 

 preserved portion, the true diameter must have been just about 2 cm. This gives 

 a proportion between M. anglica and M. pusilla of rather more than 3*5 : 1. 

 If we compare the stelar systems, the difference is somewhat greater — that of 

 M. anglica in the best-preserved specimen measuring about 4x2 cm. as 

 against 7 x 5"5 mm. in M. pusilla, taking the periderm as the boundary in 

 both cases. Prom the means, 3 cm. and 0'625 cm. respectively, we get a 

 proportion of 4"8 : 1. Eoughly, we may say that the linear dimensions of 

 M. pusilla were about one quarter of those of a typical specimen of M. anglica. 



Stelar System. 



The three steles are nearly equal in size, attaining a diameter of about 

 3 mm. Little is preserved except the wood, though here and there remains 

 of the cambium and phloem can be found. The primary wood has a 

 somewhat triangular transverse section (fig. 3). It is composed for the most 

 part of tracheides, with comparatively little xylem-parenchyma among them. 

 The smallest elements, presumably protoxylem, are found at the prominent 

 angles, very near the outside of the primary xylem, but whether the 

 structure was slightly mesarch or actually exarch could not be determined 

 with certainty ; there is evidence pointing in both directions. Similar 

 difficulties were met with in the case of M. anglica, but there, with the help 

 of the longitudinal sections, it was possible to obtain definite proof of 

 mesarch structure (Scott, '99, Plate 10, fig. 5). In M. fusilla the partially 

 longitudinal section of one stele does not clearly show the position of any 

 protoxylem group. 



The secondary wood is very unequally developed, attaining its greatest 

 thickness, about 12 elements, on the inner side of the steles (fig. 3), as has 



