430 BOTANICAL GAZETTE [may 



In cross-sections the equality of the two branches is just as 

 apparent. Fig. 9 shows a section of the central cylinder below a 

 dichotomy at a distance of about half a year's growth from the 

 two tips. The central part of the cylinder is filled with immature 

 tissue, which can be distinguished approximately into xylem and 

 phloem areas. The shaded areas represent fully differentiated 

 xylem made up on the outer side of protoxylem tracheids, and on 

 the inner side of larger metaxylem elements. As differentiation 

 proceeds in this species, the metaxylem portions meet in the 

 middle of the cylinder and become more or less continuous, and the 

 phloem occupies the indentations or bays between the xylem 

 strands. Surrounding the cylinder there is a clearly marked 

 endodermal sheath in the form of an irregular layer of cells, only 

 the inner contiguous walls of which are thickened at this stage of 



t 



9 10 ^ "—11 



Figs. 9-1 1. — Cross-sections of central cylinder of forking stem, xylem areas 

 shaded; small cross-hatched areas indicate leaf traces; X52. 



development. In fig. 11 the whole sheath is shown, but in figs. 

 9 and 10 only the thickened cutinized walls are to be seen. As 

 the dichotomy proceeds the xylem strands change position but 

 little, the sheath from opposite sides pushing inward until the 

 division into two similar cylinders is complete. In this mode 

 of branching no branch gaps of any kind are formed in the sheath 



or phloem. The length of stem between figs. 9 and 



em 



mm 



10 and n it was o.smm 



em 



that in each of the two branches there are five. The new strands 



are not formed by a division of old ones, but arise de novo out of 



meristematic tissue. The beginnings of these strands may be 

 seen in fig. 10. 



As shown by Jones (5), the protoxylem strands in Lycopodium 

 are constant neither in number, shape, nor position. New strands 



