352 PROFESSOR W. C. WILLIAMSON ON THE PLANTS 



forms of Stembergia, in which they are much thicker and 

 less numerous ; implying, of course, an inverse arrange- 

 ment of the medullary laminae. These differences are but 

 the parallels of what has been already stated to exist in the 

 recent plants, and may either depend upon the age of the 

 plant, or indicate a difference of species ; probably both. 



Fig. 10 represents the beautiful specimen from the col- 

 lection of the late Dr. Charles Phillips, for which I am in- 

 debted to John Bury, Esq. of Scarborough. The drawing 

 is two-thirds the size of the original. We have here 

 (fig. 10 a) the luore ordinary form assumed by the English 

 Stembergia approximata, surrounded by a cylinder of 

 ligneous tissue, presenting the same structure as that already 

 described; only, instead of the woody zone being little more 

 than the -^ of an inch in thickness, it here reaches half an 

 inch. The narrow space between this ligneous zone and 

 the so-called Stembergia, is filled up with pulverulent car- 

 bonaceous matter, which is obviously the mineralized re- 

 sidue of the true medullary tissue ; the structure of which, 

 in this, as in almost every other instance hitherto discovered, 

 has been lost. The exterior of the ligneous zone is smooth 

 and carbonaceous — being, in fact, the outer surface of the 

 decorticated wood. In all these points it appears to resem- 

 ble the specimens found by Mr. Dawson in Nova Scotia. 



In the exterior of the wood of fig. 1, the fibres of pleur- 

 enchyma are very distinct, even when viewed through a 

 low magnifier; and, what is interesting, there are numerous 

 points where the fibres diverge a little, meeting again lower 

 down. Vertical sections of these points show them to be 

 abortive buds and branches. In the interior of each is an 

 extension of the central medullary tissue, which, of course, 

 has not as yet assumed the discoid arrangement This 

 process is reserved for a later stage of its development. 



At the exterior of fig. 10, the carbonaceous matter 

 x)bscures the fibres; but in the fractured surfaces a very low 



