222 MR R. KIDSTON AND MR D. T. GWYNNE-VAUGHAN ON 



wall separating them appears as a delicate reticulation. In fact, in this last case they 

 might very well be described as porose. The end walls of the tracheae are marked with 

 the same pattern as the side walls. In the transverse section of this fossil the tracheal 

 walls are represented in such a manner that a considerable amount of structural detail is 

 visible in the substance of the wall itself. In its best state of preservation the tracheal 

 wall is represented by a brown-coloured substance showing up in the matrix of the 

 fossil. The brown tract is delimited from the cavities of the tracheides that it separates 

 by very fine black lines. These are usually most conspicuous at the angles and at the 

 unpitted regions of the wall (figs. 8 b, c, and d), but they are often more or less broken 

 up. and may even be absent altogether. A varying number of thin black lines also occur 

 in the median region of the brown tract that represents the wall (figs. 8 a-d). These 

 are of different lengths and are separated from one another by the homogeneous brown 

 substance of the tract in which they lie. Similar black lines also occur at the angles 

 where three or more tracheae meet (figs. 8 a-d). They may either meet at the very 

 centre of the angle to form a three- or four-rayed figure, or they may delimit one or more 

 sides of an angular mass of the homogeneous brown substance occupying that position. 

 In either case they radiate out shortly into the walls that meet at the angle, but 

 they never become continuous with the other dark lines that lie in the substance 

 of these walls. 



From comparison with longitudinal sections it is clearly seen that the longer of 

 these black lines in the median region of the wall correspond to the several vertical 

 series of pits. Thus, if there is one series of pits, there is a single long black line 

 (fig. 8 a) ; if two or more series of pits, there are two or more black lines of equivalent 

 length (fig. 8 b). Here, however, a difficulty arises, for very often a very short line is 

 also present between the longer lines which correspond to the pit series, and this can- 

 not be referred to any pit at all (figs. 8 c and d). In many parts of the fossil the walls 

 sometimes appear to have become more or less disorganised and dissolved before 

 fossilisation. This breaking down usually occurs in the middle of the wall, and affects 

 either an unpitted region or else the middle series of pits (fig. 8/). Sometimes the 

 wall is broken down at several places, or even along its whole length (fig. 8 e). In these 

 regions the brown-coloured substance is partially or entirely absent, and the wall is 

 represented by a thin black line alone. 



The peculiarities just described in Zalesskya gracilis and also others noted in the 

 species of Osmundites dealt with in Part I. of this paper were so difficult to correlate 

 with the generally accepted idea of the structure of a fern tracheide that a careful 

 re-examination of the xylem of the living Osmundacese was rendered necessary. This 

 investigation was carried out by one of us, and the results, which are here shortly 

 summarised, will be published in detail as a separate paper in a forthcoming number 

 of the Annals of Botany. Contrary to expectation, it was found that the middle 

 substance of the mature wall separating two contiguous tracheae is entirely wanting 

 in the regions of the pitted areas. In fact, the middle lamella and the primary walls 



I 



