Sinnott . — The Evolution of the Filicinean Leaf- trace. 189 
ancestors of the former family. This single strand becomes in the petiole 
a broad arch, which is continuous in the Osmundaceae and broken up in the 
Ophioglossaceae. The primitive condition of this type of trace is an ellip- 
tical concentric strand with one mesarch protoxylem (Text-fig. 1). 
3. The second type is characteristic of the Marattiaceae. In all the 
members of this family save Angiopteris , and in young plants of this genus, 
the base of the leaf-trace consists of two bundles, each of which has a single 
protoxylem. This is endarch in all except Danaea , where it is often mesarch. 
A complicated arch of bundles in the petiole develops from these two early 
ones. The primitive condition of this type of trace is two circular concen- 
tric bundles, each with one mesarch protoxylem (Text-fig. 2). 
4. The third type is characteristic of all remaining ferns, and its primi- 
tive condition is a single, roughly triangular, concentric bundle, with its base 
towards the stem-axis and with three mesarch protoxylems, one near each 
corner (Text-fig. 3). In the Schizaeaceae, Gleicheniaceae, and primitive 
Matonineae among the Simplices, the strand is single throughout, and at its 
base is always triarch and often mesarch. Lygodiuni alone among the Fili- 
cales shows a petiolar structure which is neither endarch nor arched, and which 
is doubtless very primitive. In the simpler Gradatae, the trace becomes 
broadened into a tetrarch, flat-topped arch, which becomes separated into 
many strands in the Dicksonieae and Cyathaceae. The Hymenophyllaceae 
form a reduction series from the Simplices. In the lower Mixtae, the un- 
divided tetrarch trace persists, but in the bulk of the smaller and simpler 
forms it becomes divided into two equal diarch bundles. The complicated 
petiolar system of the higher Polypodiaceae is always referable to this 
simpler type. 
5. The monarch trace may be considered as the persistence of a very 
primitive condition. The diarch type has perhaps been derived from the 
constriction and separation into two of such a primitive diarch bundle as 
that of Clepsydropsis among the Zygopterideae, while the triarch condition 
may have arisen by the amplification of a similar bundle into a tetrarch 
strand, a slight reduction of this, and the fusion of the two median proto- 
xylem groups. 
6. The constant endarch and concentric structure of the leaf-bundle, as 
opposed to the varying conditions in the stele, point to the former as a con- 
servative organ. 
7. The petiole at its attachment is in many cases very slender, and the 
base of the leaf-trace presents always its smallest and most compact condi- 
tion. Structures are often retained here which are undoubtedly very primi- 
tive. The petiole is largest at the point where it first becomes free, 
probably for mechanical reasons, and the bundle here is in its most complex 
form. 
8. Since the transpiration current seems to be accommodated equally by 
