176 Sinnott. — The Evolution of the Filicinean Leaf -trace. 
members of which has been investigated by G wynne- Vaughan (14 and 15). 
These are typically siphonostelic ferns, which both in their soral characters 
and in their anatomy are intermediate between the Schizaeaceae and 
Gleicheniaceae on the one hand, and the Polypodiaceae on the other. 
Loxsoma is clearly the least specialized and most primitive of the three 
genera. In its sporangial characters it clearly shows an affiliation with 
certain of the Simplices and possibly with the Hymenophyllaceae, but in 
the form of the indusium and in its general habit, as well as in anatomical 
structure, it is closely allied to the Dennstaedtineae. It possesses a well- 
developed exarch siphonostele from which the leaf-trace departs as a single 
arched concentric strand, concave towards the axis and with incurved ends. 
This has four endarch protoxylem groups, one in each lateral hook and one 
in each of the two angles of the arch (Text-fig. 5). In the upper part of the 
petiole these two median groups approach each other, and they finally fuse 
in the rachis. This type of trace is also characteristic of the Dennstaedtineae 
and of many of the lower Polypodiaceae. It has probably arisen from our 
hypothetically primitive condition much as has the leaf-bundle of Aneimia 
or Mohria. 
Dennstaedtia and its allies show a higher development of the sorus, 
which often displays a marked tendency towards the c mixed ’ condition 
of the Polypodiaceae. Anatomically the family is almost identical with 
Loxsoma , though the siphonostele may be mesarch as well as exarch. In 
D. punctiloba. \ as examined by the writer, the endarch protoxylems of 
the leaf seemed to be continuous with the peripheral scalariform elements 
of the stele. It would perhaps seem from this that the two distinct kinds 
of protoxylem — cauline and foliar — noted by G wynne- Vaughan in Lox- 
soma (14) might, in closely related forms at least, be really the same. 
The Dicksonieae 1 are much larger ferns than Loxsoma or the Denn- 
staedtineae, and show a close approach to a dictyostelic condition. The 
leaf-trace arises as a long and narrow band of xylem with incurved ends, 
and either in the cortex or the base of the petiole usually breaks up into a 
large arch of separate strands. Though little information is available as to 
the ontogeny and more minute histology of the trace, it has most probably 
been derived through the amplification of such a bundle as is found in Lox- 
soma , for Chandler (10) found the young trace of D. ant arctic a to be single 
and later double, a condition which we shall observe to be characteristic of 
the higher Grad at ae and of the Mixtae. 
The genus O node a, though possessing a gradate sorus and in some 
ways showing a resemblance to the Cyatheaceae, is identical in its anatomical 
structure with certain of the simpler Mixtae, and represents the next step in 
development above the Dennstaedtia condition. The cylinder is dictyostelic 
and the leaf-bundle is a double one, each of its diarch members representing 
1 The name Dicksonia is here used for the genus exclusive of section Paiania (. Dennstaedtia ). 
