PRIMARY STRUCTURE OF CERTAIN PALEOZOIC STEMS. 343 



of the former sj:)ecies represented in fig. 1 and phot. 2 ; or again, if we compare the 

 smaller xylern-strands of the two species (fig. 9 with fig. 2). In C. beinertianus, as in 

 the former species, the outgoing strand has an almost circular transverse section, with 

 the smallest elements towards the centre (fig. 8). As the strand figured is damaged at 

 the back, we cannot be quite certain whether the structure was strictl} 7 mesarch, i.e., 

 whether there were tracheides all round the protoxylem. From the evidence of another 

 outgoing strand, however, it is probable that this was the case, so that the resemblance 

 to C. fascicularis appears to be complete, so far as these larger strands are concerned. 

 I have also carefully examined all the smaller xylem-strands shown in the transverse 

 sections ; some of them, like the outgoing bundles, may be mesarch, but the majorit} 7 

 are certainly endarch ; some have a structure which may be described as hippocrepi- 

 form-endarch (see fig. 9) ; that is to say, the ring of tracheides is interrupted at the 

 back of the bundle, so that on the side towards the pith the protoxylem is in contact 

 with thin-walled tissue. There is very little real difference between this structure and 

 that of the smaller mesarch strands of C. fascicularis (cf. fig. 2) ; there also the xylem, 

 or rather the tracheal tissue, is interrupted, but not so regularly on the inner side of the 

 strand. Similar differences occur amono; the bundles in the stem of Osmunda* 



This partial assumption of endarch structure is of interest, as marking the first step 

 in that disappearance of centripetal xylem which characterises the later types of 

 Gymnospermous stem. 



In some of the longitudinal sections the spiral tracheides of the protoxylem are 

 quite distinct, as in the bundle shown in fig. 10. Here the protoxylem is adjacent to 

 parenchyma, but the poorly preserved element still further to the interior is a tracheide. 

 The section, however, as shown by the direction of the medullary ra} 7 s. was not accu- 

 rately radial, so most probably this was a ' hippocrepiform endarch ' bundle, one of the 

 flanking tracheides appearing on the inner side of the strand in consequence of the 

 deviation of the section from the radial plane. The primary tracheides show the usual 

 transitions through reticulate to pitted structure. The walls of the largest of the 

 primary xylem-elements have numerous rows of hexagonal bordered pits, sometimes 

 beautifully preserved (see fig. 11). The largest primary tracheides are as much as *1 mm. 

 in diameter ; those of the secondary wood seldom exceed "05 mm. 



The pitting of the secondary wood, usually imperfectly preserved, but well shown 

 at a few places, is limited, as usual, to the radial walls. The pits are most often in two 

 rows only ; sometimes they are scattered, and even when in contact do not usually assume 

 a regular hexagonal outline, though sometimes there is an approach to this form. The 

 bordered structure of the pit with a narrow slit-like pore is evident in the better pre- 

 served parts of the wood. Examples of medullary ra}^, as seen in tangential section, are 

 shown in fig. 12. They are nearly always one cell only in thickness; cases where the 

 ray is locally two or more cells thick (as in fig. 12, B) are very rare, and appear to be 

 connected with some irregularity in the course of the tracheides. The great differences in 



* Zenetti, Bot. Zeitung, p. 57, woodcut 2, and p. 62, woodcut 3, 1895.. 



