346 DR D. H. SCOTT ON THE 



Distinctive specific characters of C. heinertiana are to be found in the large size and 

 peculiar structure of the pith, the relatively small extent of the primary xylem, the 

 frequent endarch structure of the smaller primary strands, and the usually somewhat 

 scattered arrangement of the pits on the secondary tracheides. 



II. Pitys, Witham, emend. 



1. Pitys antiqua, Witham. 



The structure of this stem has been investigated principally in a specimen from Mr 

 Kidston's collection (sections 598a-598h), collected by Mr B. N. Peach, F.R.S., at 

 Lennel Braes in Berwickshire, in 1883. The specimen, like those already described, 

 is of Lower Carboniferous age (Calciferous Sandstone Series). Its specific identity has 

 been established by comparison with Witham's type-specimen, as will be explained 

 below. 



In this case, also, I am indebted to Mr Kidston for calling my attention to the 

 peculiarities of the fossil, and lending me his sections for investigation. 



The stem is remarkable for its large pith, which in the specimen collected by Mr 

 Peach, has a diameter of 22 mm. (phot. 5). In a section from another specimen, also 

 from Lennel Braes (No. 221 in Mr Kidston's collection) the pith as preserved is as much 

 as 34 mm. in diameter, and may be incomplete. The structure of the pith is character- 

 istic ; it consists of large, but very short cells, their width, which usually exceeds their 

 height, being from '15 to "2 mm. (see figs. 14, 15, and 16). Many of the pith-cells are 

 filled with dense carbonaceous contents, suggesting that they may have been secretory 

 elements. Although they do not differ in form from the surrounding, comparatively 

 empty cells, yet their somewhat regular arrangement, and the fact that their relative 

 frequency is unaffected by the state of preservation, may indicate that some real 

 differentiation existed during life. 



A conspicuous feature of the pith is the presence of large, horizontal, lenticular gaps 

 in its tissue (see phot. 6). These gaps are largest in the outer part of the pith, though 

 they are present to some extent all through it. They appear to be due to a shrivelling 

 of the tissues, for the cells between the larger gaps are flatter than elsewhere, and 

 have a collapsed look. The gaps extend out into the principal medullary rays. 

 Their presence gives the pith, as seen in radial section, an appearance not unlike 

 that of the well-known discoid pith of Cordaites, but much less regular. It may be 

 doubted whether the resemblance is more than a superficial one. The Cordaitean 

 discoid structure, as shown, for example, in Cordaites Brandlingi* is strikingly regu- 

 lar, and appears to have been due to rupture during a normal process of growth ; the 

 gaps are strictly limited to the middle part of the pith, the peripheral zone being un- 

 interrupted ; neither do the cells show any signs of collapse. In all these points the 

 pith of Pitys antiqua is different ; here the imperfectly discoid structure has much 



* Scott, Studies in Fossil Botany, fig. 137, A ; Eenault, Cours de Bot. Fossile, vol. i. pi. 12, fig. 12. 



