318 BOTANICAL GAZETTE [april 



chyma, although a few parenchyma cells usually remain in the center 

 as long as the leaf trace can be identified (fig. 15). For some distance 

 the compact bundle remains distinctly collateral, but the phloem 

 decreases in amount, and near the stele disappears entirely, the leaf 

 trace consisting of a strand of xylem with a few parenchyma cell- 

 in the center and a parenchyma sheath. The xylem cells become 

 shorter, and it is no longer possible to distinguish between protoxylem 

 and metaxylem. The next stage is the merging of the tracheids and 

 parenchyma of the various leaf traces to form the central axis. 



While in general the leaf traces of the other species are similar to 

 those of /. echinospora var. Flettii, they present certain interesting 

 variations. The leaf of /. Tuckermani var. Harveyi is also quadran- 

 gular, but is shorter and more rigid. The bundle, however, is not 

 so well developed, either in amount or structure. The tissues are not 

 well differentiated; the phloem in particular is much less sharply 

 differentiated than in the other three species. The thickening of 

 the walls is slight and sieve plates are not evident. The amount of 

 xylem is noticeably less, but the phloem, although not abundant, I 

 not so much reduced as the xylem. In the lower part of the leaf the 

 phloem is a narrow band, but in the upper part of the leaf it forms an 

 arc and tends to surround the xylem. The formation of sieve ce 

 begins in the middle of the band, or arc, and extends around three 

 sides of the xylem. The phloem in the center of the arc i> small in 

 amount and more rudimentary than that on either side, so t at 

 places there are indications of a tendency to form two lateral groups 

 The sieve cells in the lateral groups are smaller, as well as more 



u f t formed 



numerous, and they continue to function long after the nrsi-i 

 cells have become crushed and functionless. In this species 

 sporangia are comparatively small. , rge 



/. melanopoda has very long, moderately stout leaves, w » 

 sporangia. The leaves have a greater diameter than those o^ • 

 Tuckermani var. Harveyi, but they are very much longer and re a I ^ 



less stout. The amount of xylem is greater, and the bun e 



1 tVip leat trait- 



general stronger than in the two preceding species. 4 ™ ^^ 



in the cortex of the stem contains on an average twice *> ^ 



tracheids as that of /. Tuckermani var. Harveyi, while in 



trace above the sporangium there may be three to eiffht tra 



