THE ANATOMY AND AFFINITY OF PLATYZOMA MICROPHYLLUM. 645 



pericycle, and the phloem is a narrow band displayed over the lower convex surface 

 of the xylem and around its hooked margins. The xylem is an extended narrow 

 strap, which is composed of two types of elements. The lower, central, and more 

 massive portion is composed mainly of large tracheides. The incurved margins 

 of the strand are of numerous, very small, and relatively thin-walled tracheides. 

 This marked difference in size between the elements of the central and marginal 

 portions of the xylem is a conspicuous feature throughout the greater portion of 

 the leaf. The incurved xylem-strap encloses a parenchymatous ground tissue, and 

 if protoxylems are at all developed they are involved in the small-celled and arched 

 margins of the strap. As the rachis is approached the petiole becomes somewhat 

 flattened dorsiventrally, and this is accompanied by a lateral extension of the 

 trace. 



Within the rachis itself little or no change takes place in the epidermal and 

 cortical tissues, but the leaf-trace has become extended laterally, and the margins 

 of the xylem-strap have become thinner and more attenuated (fig. 15). The pinna- 

 traces are small, and of extra-marginal origin. They are given off from the 

 convexity of the incurved leaf-trace, their xylems being contributed by the sharply 

 curved parts of the small-celled and incurved margins of the xylem-strap. The 

 pinna-traces pass quickly through the cortex to the almost sessile pinnae. The 

 venation of the pinnae has already been indicated. Towards its distal end the 

 rachis becomes slender and laterally compressed. The xerophytic characters of 

 the superficial and cortical tissues already noted are still dominant, and the leaf- 

 trace, though now greatly reduced in bulk, retains the general features noted 

 in the lower regions of the leaf (fig. 16). The pinna-traces are still of extra- 

 marginal origin, but now the drain made by the pinnae upon the sharply curved 

 portion of the xylem-strap leads frequently to a depletion of the small tracheides ; 

 so that, when seen in section, the margins of the xylem-strap appear fragmented, 

 and separated from the main lower and central portion of the strap. The margin 

 of the xylem-strap is, in fact, in process of reduction. Close to the apex of the 

 leaf the rachis has become very slender. Its cortex is greatly reduced, and the 

 leaf-trace, though now very small, is still clearly defined (fig. 17). The infolded 

 margins of the xylem-strap have been entirely reduced at a lower level, and the 

 strap has become a thin, irregular, and disintegrating assemblage of tracheides, 

 from which the last few small pinnae cannot fail to be supplied in a marginal 

 manner. From what has been recorded, it will be evident that in general form 

 of leaf-trace, and in the dominantly extra-marginal origin of the pinna-traces, the 

 pinnate leaf of Platyzoma falls easily into line with most Gleichenioid types. 



From the section of a small pinna shown in fig. 17, it will be clear that the 

 epidermis is well developed, and that the mesophyll of the pinna is partly lacunar. 

 The upper epidermis is strongly cuticularised, and is devoid of stomata. The 

 stomata are restricted to the lower surface, and lie hidden within the almost 



