OF LOWER GREENSAND PLANTS. 289 



cells with thickened walls, in which true vessels arc not 

 recognisable. As the radial sections do not pass through the 

 primary wood, the nature of the protoxylem-walls cannot be 

 determined. In the secondary wood the vessels are often but 

 little larger than the wood-fibres of the radial series in which 

 they lie (text-fig. 90). They are generally roughly circular, 

 with slightly flattened sides. The larger vessels are a little 

 more irregular, but also roughly circular, and average about 

 20-40 /x, principally about 30 /* in diameter. The walls are 

 thickened, but not remarkably so, the lignified walls thinner than 

 those of the adjacent wood-fibres. The majority of the walls have 

 scalarifonn perforations ; locally these may merge into scalari- 

 ibrm-like oval pits, and here or there, finally, into roundish and 

 more irregularly placed pits. The assumption of Janssonius and 

 Moll (1912, p. 625), that the scalariform pitting is only on the 

 end-walls, is incorrect, as also is their statement that the end- 

 walls arc " very obliquely placed." Where they can be seen in 

 the actual fossil, the end- walls seem to be placed at a rather 

 unusually high angle, thus giving the vessels blunt instead of 

 very elongated ends. 



The wood-fibres (fibre-tracheids) vary considerably, but on the 

 whole tend to be squarish or rectangular, averaging 15 x 10 /i in 

 diameter, a few nearly 20 p. The radial is generally less than, 

 sometimes only half, the tangential diameter. Walls greatly 

 thickened, so that the lumen is only one-third or less of the 

 whole diameter (text-fig. 90). The walls are perforated by 

 numerous round bordered pits, in both the tangential and radial 

 walls. These are seen remarkably well in transverse section 

 (text-fig. 90, a.), where a single section of a cell may show as 

 many as four pits, one in each of its walls. Vertically, the pits 

 lie in a single row or in two rows here and there, and each pit 

 in the radial walls is separated from its neighbours roughly by 

 its own diameter. 



The medullary rays consist of elements all of which have 

 thickened walls with numerous simple pits. The cells are all 

 closely adjacent, entirely without intercellular spaces. The 

 various cells differ considerably in shape and size (text- 

 figs. 91, 92). Some being nearly square in radial, and hexagonal 

 in tangential section (w.), others being rectangular and elongated 

 either vertically or horizontally (r. and *.), while the end-cells 



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