THE ANATOMY AND AFFINITY OF PLATYZOMA MICROPHYLLUM. 639 



pinnate leaf, and the smaller filiform leaf, may be regarded as harkings back to 

 steps taken earlier in an evolution of reduction, from which the unbranched pinnate 

 leaf and the larger filiform leaf have emerged as dominant types. The anatomy 

 of the various leaf types recorded can be considered when we have before us 

 the main facts concerning the anatomy of the rhizome. 



It will be convenient to delay the consideration of the sporangia until the 

 end, and accordingly the examination of the chief anatomical features of Platyzoma 

 will now be made. 



The axis of Platyzoma possesses a broad cortex, developed peripherally as a 

 strong zone of sclerenchyma, and more centrally as a mass of storage parenchyma 

 (fig. l). The stele is centrally placed, and from it depart a close succession of 

 leaf-traces and roots, which pass out rapidly through the cortex and emerge in 

 close succession upon the surface. The leaf-traces, on their way out through the 

 cortex, are oval or almost circular in transverse section, and vary in size from 

 large traces, originating mainly from the upper surface of the stele, to smaller 

 traces, which arise from the sides and lower surface of the stele. The majority 

 of the roots come off from the lower surface of the stele, but occasionally they 

 have a lateral origin. There is a bulky central sclerenchymatous pith. Its cell 

 walls are not, however, strongly thickened, and centrally they are of a light brown 

 colour, and enclose mucilaginous protoplasts. The pith is surrounded by an un- 

 interrupted endodermis, which is immediately followed by two or three layers 

 of large-celled parenchyma. There is no inner phloem, and the large-celled 

 parenchyma is followed immediately to the outside by a remarkable ring of 

 xylem. The xylem-ring varies in thickness as one passes from the upper to the 

 lower surface of the stele. On its upper surface the ring is invariably thin, while 

 on its lower surface it is relatively thick. The thickness of the xylem-cylinder 

 increases gradually during the passage from the upper to the lower sides of the 

 stele. At some points along the course of the rhizome this difference in the 

 thickness of the upper and lower portions of the stele is only slightly marked 

 (figs. 4 and 10), while at other points it is a very conspicuous feature (figs. 1, 13, 

 and 63). The bulk of the sclerenchymatous pith does not, however, vary to any 

 great extent from point to point, for increase in the thickness of the lower side 

 of the xylem is not attained so much by encroachment centrally, as by a centrifugal 

 thickening. 



The xylem-cylinder is composed of scalariform tracheides together with chains 

 and groups of parenchymatous cells, which follow irregular courses between the 

 tracheides, and which fuse and branch repeatedly. No protoxylem elements can 

 be recognised, but the xylem-cylinder can be divided roughly into two concentric 

 cylinders according to the characters of the tracheides and the distribution of 

 the parenchyma. The inner cylinder is composed of wide storage tracheides 

 associated with numerous chains and masses of # parenchyma. It forms, essentially, 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 20). 94 



