THE ANATOMY AND AFFINITY OF PLATYZOMA MICROPHYLLUM. 649 



pair. This seems to be due mainly to a thickening of the extraordinarily extended 

 leaf-trace margins, at the points where the infolding of the margins takes place. 

 This thickening of the margin is more pronounced in fig. 36, which shows the 

 extra-marginal departure of the fifth pinna-trace. When this last pinna-trace had 

 departed, the xylem became suddenly a solid mass of tracheides (fig. 37). It is 

 probable that this marked the approach of the leaf-tip, and that few if any more 

 pinnae were developed on this leaf. 



And lastly, in the pinnate portion of the small leaf which is represented in fig. 

 12 — and which showed the simplest possible pinnate condition — the supply to the 

 pinnae is of extra-marginal origin. Three stages in the preparation of the pinna-traces 

 of this leaf are shown in figs. 38, 39, and 40. The condition typical of the trace 

 in the rachis of the pinnate leaf, and in the middle region of the larger filiform leaf, 

 was found to dominate the stalk. It is depicted in fig. 38. Fig. 39 shows the 

 lateral extension of the trace as the pinna-traces begin to form, and in fig. 40 one of 

 the two pinna-traces is almost liberated. 



The facts regarding those various leaf-types have been recorded in some detail ; 

 for not only was it necessary to recognise the transitions from pinnate to filiform 

 leaves by external form, but a knowledge of the anatomical features could provide 

 a valuable check on any conclusions which might be drawn. On the ground of 

 external form, it seemed almost certain that the large filiform leaves might 

 justifiably be regarded as reductions from pinnate leaves, and the evidence which 

 has now been advanced — both regarding these filiform leaves themselves and 

 also the "transitional" types — seems to lend strong support to this opinion. It 

 is not to be supposed that the "transitional" types are considered illustrations 

 of definite transitional forms, lying between the departure from the pinnate 

 leaves and the arrival of the filiform leaves, but in themselves they are proofs of 

 leaf-reductions in Platyzoma, and the filiform leaves may be resultants of a like 

 reduction process. 



The sporangia remain to be considered. At present I am unable to give an 

 account of their development, but there are certain outstanding features which 

 may be noted here. They are developed either terminally upon the main lateral 

 branches of the sympodium, or, if more than one sporangium is developed upon 

 a branch of the sympodium, the distal portion of the branch is forked and the 

 sporangia are again terminal (text-fig. 9). The bundle-endings which thus 

 supply the isolated sporangia are worthy of note. They are composed of long 

 tracheides with spiral and reticulate markings, and a sheath of long, narrow, and 

 delicate cells which persist to the very tip of the bundle (text-fig. 10). I am 

 not aware of any other Leptosporangiate Fern in which the condition just described 

 exists. It may be most nearly matched by some early Eusporangiates. The 

 sporangia in any one pinna vary in size, but no definite arrangement of the 

 sporangia according to size can be followed. There are large and small sporangia, 



