Lady Isabel Browne. 
18 
Dr. Campbell urges that the similarity between the develop¬ 
ment of the reniform leaves of some Hymenophyllaceae and the 
leaves of the Salviniacere, and the absence of secondary roots in 
species of Trichomanes and Salvinia suggest an affinity between 
these two orders (13). The latter resemblance is clearly due to 
parallel development, for the less reduced species of both genera 
possess secondary roots. The fronds of both orders are reduced, 
though in different ways ; the relatively thick leaf of the Salviniaceae 
could hardly be derived from a filmy frond, since it is exposed to an 
even greater extent to the damp conditions that induce or are 
associated with the filmy habit. There is, however, a certain 
resemblance in the structure of the sori of the two orders and in 
their endarch protostely. Even if the Hypienophyllaceas are 
primitively exarch the Salviniaceae may have been derived by 
reduction from the modified endarch forms. But they may equally 
well have lost their definite and originally peripheral protoxylem 
owing to slow growth and reduction of the stem, the xylem then 
developing irregularly and somewhat centrifugally. 
It is hardly necessary to point out that the Salviniaceae show 
little, or no, resemblance to the Schizaeaceae. In respect of the 
soral distribution of their sporangia the Gleicheniaceae come slightly 
nearer to the Salviniaceae, but even they are very different. The 
Matonineae, Cyatheaceae, Polypodiaceae, Loxsomaceae and Osmun- 
daceae are all, in their various ways, so complex, and Salvinia and 
Azolla are so reduced, that it seems hopeless to trace any close 
affinity between the two latter genera and the other orders. Thus, 
though the Salviniaceae clearly originated from homosporous lepto- 
sporangiate Ferns, their exact origin is obscure. 
MaRSILEACEvE. 
The Marsileaceas are reduced, heterosporous, solenostelic 
Ferns. Mr. Johnson claims that the “ sporocarp ” enclosing the 
sori is of indusial nature, and that it and its stalk are “ homologous 
with the petiole only of the sterile branch of the leaf ” (25), (26). 
Dr. Campbell regards the sporocarp as made up of infolded and 
coherent leaflets (13). Mr. Johnson relies chiefly on the sequence 
of divisions in the segments of the apical cell, but this does not 
appear to be a reliable index of morphological value (9), (35). 
Further the branching and course of the vascular bundle of the 
peduncle on entering the sporocarp suggest that the latter is a lobed 
leaf rather than an indusium (13), (34). 
The most important point in which the Marsileaceas recall some 
Botryopteridese is the branching of the fertile frond in a dorsi- 
