652 MR JOHN M'LEAN THOMPSON ON 



Summary. 



The chief facts obtained from the single herbarium specimen available of 

 Platyzoma have now been described. At some points the account has been less 

 complete than would have been the case if sufficient material, properly preserved, 

 had been at hand. Development has been necessarily omitted. But the fuller facts 

 relating to mature structure which have been acquired may help towards establish- 

 ing the position which this plant should hold in relation to other ferns. The habit 

 of the plant is chiefly notable in relation to the heterophyllous character of its 

 xerophytic foliage. The horizontal rhizome bears with some degree of regularity 

 zones of smaller — probably reduced— foliage leaves with pinnse vestigial or absent, 

 and these alternate with zones of larger pinnate leaves which are mostly fertile. 

 The rhizomatous condition is probably derivative from one. in which the axis had an 

 upright habit, and the heterophylly has probably followed upon the change of habit. 

 Occasional cases of bifurcation of the leaves have been seen. It is probable that 

 these are of the same nature as those branchings seen in the " forma fur 'cata" of 

 many Ferns. The crowded grouping of the leaves is a feature in strong antithesis to 

 the isolation of the leaves prevalent in the Gleichenias , with which Platyzoma has 

 habitually been grouped. The presence of hairs and absence of flattened scales 

 accords with what is seen in Gl. linearis and pectinata, and is again a point marking 

 Platyzoma off from many of the Gleichenias ; while it is in itself a character of 

 relatively primitive Ferns. But external habit, upon which the systematists have 

 hitherto placed their chief reliance, has sunk in importance before the remarkable 

 characters of the stele, the leaf-trace, and the sporangia. 



The stele is little removed from the protostelic state. It consists of a continuous 

 ring, which has never been seen to be broken by leaf-gaps or perforations. Definite 

 protoxylem has not been recognised. But the xylem is differentiated into two 

 zones, an outer consisting of smaller tracheides with occasional parenchymatous 

 groups and chains, the inner of large storage-tracheides intermixed with larger 

 groups and chains of parenchymatous cells. The inner zone appears to be primarily 

 a storage zone. Centrally lies a bulky pith which is sclerotic and mucilaginous. It 

 is surrounded by a continuous inner endodermis which is throughout distinct and 

 separate from the outer endodermis. Between the inner endodermis and the xylem- 

 ring lies a narrow zone of parenchyma, but no phloem. Externally to the xylem-ring 

 lies a continuous narrow zone of phloem, a pericycle, and external endodermis. 



The leaf-traces depart without any leaf-gap. They vary in size according to the 

 dimensions of the leaves which they serve, but even those of the largest fertile leaves, 

 though they may interrupt the xylem to its inner limit, do not disturb the inner 

 parenchymatous zone or the inner endodermis : thus there is no leaf-gap. The leaf- 

 trace draws off from the outer zone of xylem as a crescentic mass of tracheides, 

 accompanied by an outer arc of phloem. There is no definitely constituted proto- 



