OF LOWER GREENS AND PLANTS. 253 



tuclinal section these elements are also conspicuous (PI. XXIV> 

 &., and text-fig. 75) and very long. In tangential section their 

 narrow, sloping, pointed ends can be seen from time to time, as 

 well as their pitting. In a few cases in the sections, the walls 

 of these elements are so cut through that a surface-view of the 

 roundish or oval pits liberally scattered all over them can be 

 seen . 



(b) The thinner-walled elongated elements (sieve-tubes?) lie 

 always in radial pairs, alternating on the radius with single 

 thick-walled elements (text-fig. 73). They have the same 

 tangential diameter as the thick-walled elements (40-50 ju), but 

 are radially narrower, the pair of them together being very 

 little wider than the single thick-walled elements. While their 

 walls appear thin in comparison with their excessively thickened 

 neighbours, they are actually considerably thickened and their 

 radial walls are conspicuously pitted (text-fig. 75, and PI. XXIV). 

 In the radial section the two elements, lying between the 

 dark walls of the fibres, can be very clearly seen. In each 

 element is a single row of large pits, generally isolated and 

 nearly circular, in some cases lying in vertical pairs. The 

 diameter of the pit is nearly as great as the radial diameter of 

 the cell, so that the pit fills the wall. I have not been able 

 to detect any border to these pits. On the other hand, the 

 appearance, rather suggestive of a " sieve- area " seen in the 

 photograph, is due to minute granules in the matrix, to be seen 

 elsewhere all through the fossil. It is probable that these large 

 simple pits represent sieve-areas of which the finer pitting has 

 been eaten out, though their appearance is very like that of pits 

 in wood- elements. 



(c) The parenchyma-cell* lie only in the angle between four 

 of the last- described elements, but are either not present in every 

 case or are frequently collapsed, for the larger elements often 

 meet so as to leave only a small lacuna between them. In 

 a few cases these parenchyma-cells, much flattened, stretch 

 tangentially between a pair of sieve-tubes. When present, 

 the parenchyma-cells usually appear as squarish blackened 

 blots in transverse section (text-fig. 73, ., and PI. XXIII, 

 fig. 2), but they are much more conspicuous in the radial 

 longitudinal section, where they may lie in a vertical row 

 between the pitted elements (see a. between v. } and v. 2 , 



