348 DR D. H. SCOTT ON THE 



out into the secondary wood a little higher up the stem. The gap shown at l.t. on 

 diagram 5 (see also phot. 5), clearly marks the course of another outgoing leaf- trace. 

 The strand 23 also seems to be double, but here the tissue is damaged. From the 

 somewhat slender evidence available, it seems probable that contact between a primary 

 strand and the secondary wood only occurs at points where the former is about to pass 

 outwards, presumably on its way to a leaf. It also appears that, at the point of exit 

 from the pith, the leaf-trace was a double strand, but its two branches, as we shall see, 

 re-united in passing through the wood. Double or paired strands may also occur 

 elsewhere, independently of outgoing leaf-traces (see, for example, phot. 8, represent- 

 ing strand 38 in diagram 5). The diagram is taken from a section higher up the stem, 

 where the two strands have approached nearer to each other. 



Before further considering the distribution of the xylem-strands, it will be well to 

 describe their structure. The strands, as already mentioned, are small, indeed very 

 small in comparison with the size of the pith. Most commonly their maximum 

 diameter is about - 25 mm. ; the sectional form of the strand is usually elliptical, the 

 major axis lying in the tangential direction. A good typical example is shown, in 

 transverse section, in fig. 15, which represents the strand numbered 3 in diagram 5. 

 The smallest elements (px) lie near the middle of the strand ; towards its periphery 

 the tracheides become considerably larger, about equalling the innermost elements of 

 the secondary wood in size. A few parenchymatous cells occur among the primary 

 tracheides, especially near the middle of the group. Phot. 8 shows the same structure 

 in each strand of the paired bundle numbered 38. Of the two strands shown in phot. 

 9, one (No. 14) shows the usual arrangement of the elements; the other (13a) is less 

 regular. Longitudinal sections show the nature of the elements. Fig. 16 is from a 

 tangential section (shown as a whole in phot. 7) which passes through the periphery of 

 the pith, and here cuts through the middle of a xylem-strand. The more central 

 tracheides have an evident spiral thickening ; the narrowest among them are no doubt 

 the actual protoxylem. The peripheral tracheides of the strand are larger, and their 

 cells definitely reticulated, the lines of reticulation having a spiral course (fig. 16, r. t.). 

 Close examination shows that, in the outer tracheides, each mesh is bordered, so that 

 the reticulation is passing over into a system of spirally arranged, bordered pits. 

 Similar elements, one of which is shown in detail in fig. 1 7, occur at the inner margin 

 of the secondary wood, where, however, no true spiral tracheides have been detected.* 



Fig. 16 also shows the xylem-parenchyma, with possible secretory sacs, within the 

 primary bundle. 



The evidence thus indicates that the primary xylem-strands of Pitys antiqua have a 

 mesarch structure, differentiation having begun at a point near the middle of the strand, 

 as indicated by the position of the spiral tracheides. The mesarch structure holds good 



* I fiiid that perfectly similar elements have been described by Rothert, in recent Conifers, under the name of 

 " Gemiscbte Gefasse." See his " Tracheiden u. Harzgange im Mark von Cephalotaxus-Arten," Ber. d. Detttsch. Bot. 

 Gesellsch., Bd. 17, 1899, p. 284. 



