90 
Ethel N. Thomas. 
sprang. It is therefore not surprising that neither the Carboni. 
ferous Ferns (Botryopterideae and Psaronius), nor the supposed 
primitive families (Hymenophyllaceae, Gleicheniaceae, Schizaeaceae, 
Osmundaceae) among living Ferns show an obvious double trace. 
Seedling stages of these families are, however, very little known. 
On the other hand the early leaves of the Ferns roughly classified 
as Polypodiaceae—so far as they have been examined 1 —possess a 
double leaf-trace. The later leaves may have a four-fold trace, 
while the first one or two leaves usually show a single strand. The 
members of the Marattiaceae and Ophioglossaceae (Ophioglossmn 
not known) all show two separate bundles, or a twin strand, at any 
rate, in their early leaves. The twin strands in the first leaves of 
Botrychium are strikingly like the “ double bundles ” of the coty¬ 
ledons of Angiosperms. 
It is just possible that the double trace of the Ferns has no 
direct connection with that of the Cycado-Filices. In the Ferns it 
is found in its most marked form in the advanced rather than in 
the primitive families. This may prove explicable, particularly 
when more information is available concerning the early stages. 
However this may be, it is very striking to find the character so 
prevalent in both groups. 
The fairly universal presence of the double leaf-trace through 
the Fern-Gymnosperm series, and in a reduced form through the 
Angiosperms; together with its very frequent occurrence in 
modern Ferns, points to the existence of a common cause of a 
fundamental and primitive character Dichotomous branching at 
once suggests itself, as a primary and wide-spread phenomenon 
likely to have a bearing on the double leaf-trace, which is neither 
more nor less than an early dichotomy of the vascular system of 
the leaf.- Dichotomy is the characteristic mode of branching of the 
lower plants, and is widely recognised as the basis of many of the 
more complicated pinnate methods seen in the higher plants- 
Looked at in this way, the double leaf-trace appears as the vascular 
bifurcation corresponding to the dichotomous branching of the leaf. 
This dichotomy is obvious in the first leaves of many Ferns, but 
1 Chandler. Ann. of Bot., XIX., 1905. 
2 Professor Lignier’s very interesting paper (“ Equis. et Sphen. 
Leur origine filicineene commune.” Extrait du Bull, de la 
Soc. Linn, de Normandie. 1906) on the subject of primitive 
dichotomy—only recently come to my notice—has been of 
great service in the final formulation of this very similar 
hypothesis resulting from the seedling investigations described 
above, and which has been in process of elaboration for the 
last year or two. 
