XXXVII] OLEANDRA. ARTHROPTERIS 23 



sonioid Ferns generally shows that in allied forms the marginal position is 

 so tenaciously retained as to make it a reliable comparative feature, notwith- 

 standing the exceptions. Instability in certain clearly derivative genera does 

 not nullify the effect of constancy in many other related genera which are 

 clearly primitive. Moreover, a comparison of the leaves where the sori are 

 intra-marginal with those where they are marginal indicates that the former 

 usually show relatively broad leaf-areas of condensation, while the latter for 

 the most part retain their primitive cutting into narrow segments. In fact, 

 widening of the leaf-area and shifting of the sorus from the margin inwards 

 are features frequently related one to another. 



Oleandra Cav. 



This is a tropical genus of some ten species, with creeping and climbing shoots. It is 

 usually placed in relation to Nephrolepis, and this may be accepted provisionally. It also 

 has a kidney-shaped indusium covering sori superficially resembling those oi Nephrodiuui. 

 Until it has been fully investigated it must be ranked as a "genus incertae sedis." The 

 difiiculty in placing it illustrates once more how similar may be the results of homoplastic 

 development among the later derivative types of the Filicales. 



(ii) Eliniinatioji of the loiuer Indusium 



It has been seen how in Hypolepis the lower indusium may be partially or 

 even completely aborted in sori which have become superficial, while the 

 upper indusial lip is merged into the expanse of the blade. A similar state, 

 resulting in an apparently unprotected sorus, appears in certain other Ferns 

 ascribed by systematists to a Davallioid affinity. An example is seen in: 



ARTHROPTERIS J. Sm. 

 This is a widespread genus, but chiefly it inhabits the Old World, com- 

 prising four species. It has been placed sometimes with Polypodiuvi, or with 

 Dryopteris : but latterly with the Davallioid Ferns, in relation to NepJirolepis, 

 with which in point of habit it has much in common. The rhizome bears 

 scales, and has a vascular structure not far removed^ from solenostely. Two 

 meristeles are seen in the transverse section, while the leaf-trace departs 

 undivided, as is shown in a dry specimen in Kew. The punctiform sori are 

 intra-marginal, and terminal on the tertiary veins. A kidney-shaped indusium 

 is sometimes present, but it may be reduced, or even absent. These Ferns 

 may be regarded as partially or completely ex-indusiate Davallioids {Nat. 

 Pflanzenfam. I, 4, Fig. no, p. 206). It thus appears that elimination of the 

 lower indusium has happened also among the Davallioids, while the upper 

 indusium is liable to be incorporated in the expanse of the leaf-blade, the 

 change being essentially like that in Hypolepis, but starting from a different 

 source. 



