Sinnott. — The Evolution of the Filicinean Leaf -trace. 1 8 1 
variety of complication, but It seems reasonably clear that all of these have 
been derived, as was that of Pteris aquilina , from the simple double strand 
which we have called the Onoclea condition. 
Two genera, Woodwardia and Aspidium , in which large and rather 
complex petiolar bundles occur, were investigated as to the history of the 
foliar strand. In W. virginica the trace is an arch of from six to ten 
bundles, of which the terminal or adaxial ones are by far the largest. It is 
interesting to note that in this case, as in Pteris , almost all the bundles are 
mesarch at their base. No young plants of this species were available, but 
as the other native species, W. areolata , which is much smaller, showed on 
Investigation the presence of the typical Onoclea leaf-bundles, it seems 
reasonable to suppose that the more complicated condition has in this case 
been derived from the simpler one. Perhaps the two large adaxial strands 
in W. virginica correspond roughly to the two bundles which it originally 
possessed. 
In the case of Aspidium and the closely related genus Polys tic/mm, the 
larger native species, such as A. spinulosum , showed a petiolar bundle very 
similar to that of Woodwardia virginica , namely an arch of bundles, con- 
cave towards the axis and with two large terminal members (Fig. 21). In 
the smaller species, however, such as A. Thely pteris, the typical Onoclea 
condition was again found to be present. The young state of some of the 
larger species, such as A. cristatum and A. spinulosum , were examined and 
found to possess at first a single bundle, but later a double one throughout 
the leaf (Fig. 22). Each strand was very small and had no clearly defined 
protoxylem, but smaller cells were seen to be clustered at either end. Even 
at a very young stage small bundles began to be cut off from the abaxial 
ends of the traces and made the beginning of the later arch. In every 
case, however, both in young and in mature plants, the simplest condition 
of the vascular supply and the smallest number of bundles were always found 
at the very base of the trace. Each of the two large strands in the mature 
petiole possessed a hook and a protoxylem group only at its adaxial end, the 
other one being very obtuse. It seems a fair inference, especially from 
what we have seen as to the origin of the mature petiolar system of Cyathea 
(Text-fig. 9), that the abaxial protoxylem group of each strand, with its 
adjacent tracheides, has gone to form half of the band of small bundles which 
connects the two large ones. It thus appears that in Aspidium , as well as 
in Woodwardia , the primitive condition of the foliar supply is the double 
Onoclea bundle. 
Chandler (10) describes the vascular system in many of the seedlings 
of these Polypodiaceae, and finds the leaf-trace to be at first a single strand 
which later divides into two, a condition which is usually retained for 
a long time. In Nephrolepis cordifolia , the only species In which the 
protoxylem is figured, the leaf-trace in its early state is a single strand 
