XXIV] COMPARISON 215 



alternate with zones bearing narrow linear sterile leaves of much smaller 

 size. These betray their nature as arrested leaves of the same type as the 

 fertile by the fact that abortive pinnation may be found at the apex of 

 some of them. It thus appears that Platyzoma is a specialised Gleicheni- 

 aceous type. 



The prevalence of protostelic structure in the Family is an index of its 

 relatively primitive position, and compares with the like state in Lygodiuiii 

 which is also a genus with unlimited growth of the leaf-apex: and like it 

 the C-shaped meristele of the petiole is liable to contraction so as to present 

 a pseudo-stelic appearance. This is in accordance with the elongated 

 straggling or climbing habit. The greatest interest, however, centres in the 

 advance to typical solenostely of the axis, as seen in G. peciinata, which is 

 a species specially advanced also in the type of its sorus. The anatomical 

 facts provide material of special value for comparison with the Cyatheaceae. 

 The partial advance of the stele oi Platyzoma towards solenostely has as its 

 special interest the absence of any foliar gaps, a fact to be related to the 

 relatively small size of the leaf-trace, and the slight structural disturbance 

 caused by its departure. This provides a pointed comment on the undue 

 importance attached by some to the foliar gap, and the so-called "phyllo- 

 siphonic" state. This now appears to be simply a consequence of the relative 

 size and influence of axis and leaf, and not a characteristic of the Class. 

 Platyzoma presents in fact the anomaly of a solenostelic but not a phyllo- 

 siphonic type of Fern. 



The dermal appendaj;es of the Gleicheniaceae are very various. They 

 may appear as soft, simple or branched hairs, as stiff bristles, or as more or 

 less flattened scales (ramenta). These are often perched upon massive 

 emergences of tissue, which probably correspond to those spines of the 

 Cyatheaceae which persist as their prickly "armature." They are specially 

 prevalent in both Families about the leaf-base. It is less clear in this 

 Family than in any other what are the phyletic relations of these several 

 appendages. But in Vol. I, Chap. XI the conclusion followed inductively 

 from facts relating to many early Families of Ferns that the linear hair is 

 a relatively primitive feature, and the flattened scale derivative. Any 

 difficulty in respect of the evidence yielded by the modern representatives 

 of this essentially Mesozoic Family cannot be accepted as invalidating the 

 conclusion derived from a wide induction from the facts in other Families. 

 Many of its living species are pronounced xerophytes, and this habit is 

 recognised as promoting the formation of protective ramenta. 



Hairs only, and no ramenta, are found in the two outstanding species, 

 G. pcctinata, and linearis, as well as in Platyzoma. In the first of these the 

 hairs take the form of long stiff bristles, with a tuft of shorter divergent 

 spines radiating from the base, each being seated on a massive emergence 



