646 MR JOHN M'LEAN THOMPSON ON 



closed cavity formed by the infolding of the membranous leaf-margins. Hairs 

 arise from the leaf-margins and along the course of the veins, and assist still 

 further in the protection of the stomata. The anatomy of the pinnate leaf has 

 revealed characters expressive of extremely xerophytic life conditions. 



It will be convenient to record, at this point, the anatomical features of the 

 forked pinnate leaf which I had at my disposal (text-fig. 8). It forked near to its 

 base, no pinnse appeared below the point of forking, and the leaf-branches were of 

 equal length. The pinnate portions of this leaf were of no special interest, for they 

 agreed in general plan with the rachial part of the simply-pinnate leaf. The 

 branching petiole was, however, of considerable interest, for its trace was found to 

 undergo a perfect dichotomy. The leaf-trace — in transverse section, — as seen in 

 the leaf base, is shown in fig. 28. There is not so marked a difference in size of 

 element between the central and marginal portions of the xylem-strap as existed 

 in the simply-pinnate leaf, but the general plan of construction of leaf-trace is the 

 same in both cases. The leaf-trace of the dichotomising leaf was larger than that 

 of the simply-pinnate leaf, and its xylem was "more extended. As the petiole of the 

 forked leaf was ascended, a small group of tracheides appeared in the parenchymatous 

 bay enclosed in the curved xylem-strap (fig. 29). At a still higher level, the trace 

 had become extended laterally, and the island of tracheides in the bay of parenchyma 

 had grown considerably (fig. 30). Very soon a junction was established between 

 the island and the middle portion of the much extended xylem-strap (fig. 31), and 

 two promontories had developed — -one to right and the other to left of the free end 

 of the original island, — so that two parenchymatous bays were now formed on the 

 upper or adaxial side of the leaf-trace. A split then developed in the middle region 

 of the xylem-strap (fig. 32). It traversed the strap from its upper to its . lower 

 surface, and divided it into two similar portions. Further on, when the endodermis, 

 pericycle, and phloem had become nipped across along the same direction-line, there 

 were formed two similar leaf- traces (fig. 33) which supplied the two limbs of the 

 dichotomised leaf. It would be of interest to know what takes place in the branch- 

 ing of the leaf in the two specimens already referred to as belonging to Kew 

 Herbarium. It seems probable that they too are dichotomous branchings, but their 

 further interest lies in the facts that they are pinnate below the points of branching, 

 and that they branch at points considerably removed from their bases. It is doubt- 

 ful whether these bifurcations of the leaf of Platyzoma will bear any phyletic inter- 

 pretation. The bifurcations are seen to occur at various levels, and seem to have 

 no particular relation to the pinnae. They are probably of the same nature as those 

 bifurcations found in many other Fern leaves, e.g. Polypodium vulgare, Scolopendrium, 

 Neph rodium molle, etc., and are akin also to those distal forkings which give rise in 

 varying degree to the " forma furcata" varieties. The remarkable point is that the 

 forking should occur in such a Fern as Platyzoma, for it is more usual in advanced 

 Leptosporangiates. But nevertheless it is seen in Osmuuda. 



