tgr1] SINNOTT—FOLIAR BUNDLE 259 
lar in cross-section, and with one median mesarch? protoxylem 
group. This condition also occurs at the base of the leaf trace 
in the Osmundaceae and Ophioglossaceae, and it apparently is 
present at the node of the Botryopterideae (1), and therefore seems 
to have been the primitive one for these families. It is very evi- 
dently the earliest type of foliar strand in vascular plants, and has 
persisted almost unchanged in the microphyllous Lycopsida, but 
in the megaphyllous Pteropsida has undergone extreme modifica- 
tion. 
A very ancient type of leaf bundle has recently been investi- 
gated by BERTRAND in his account of the anatomy of the leaf in the 
Zygopterideae (1), one of the three families of the Botryopterideae, 
those simple protostelic ferns which clearly stand close to the 
ancestors of the modern Filicales. The early condition of the leaf 
trace in this group is of importance as representing the primitive 
one for all ferns. This has been found by BERTRAND in the simpler 
forms of the Zygopterideae, such as Clepsydropsis and A sterochlaena. 
These genera possess a leaf bundle which is concentric and rather 
elliptical in shape, and which contains at either end an island of 
parenchyma imbedded in the xylem. Around the island are placed 
the protoxylem elements, which are irregularly scattered in the 
former genus and in two distinct groups in the latter. BERTRAND 
has established a very complete series of forms between this type 
of leaf strand and the more complicated bundles of the remaining 
Botryopterideae, all of which seem clearly to have been derived from 
the simple diarch and mesarch condition. The original monarch 
Structure persists at the base of the trace. 
Tn a recent paper (9) the writer has endeavored to show how 
the leaf bundle of the modern Filicales has been developed from that 
of these primitive Zygopterideae. Aside from the change in shape 
and the breaking up of the strand into separate bundles, the most 
notable difference between the modern forms and their ancestors 
* The term “mesarch,” as used in the present paper, is descriptive of any pro- 
toxylem group, whether situated at the axis of the stem or not, which is surrounded 
by metaxylem. “Endarch” describes any condition where the protoxylem is not at 
the axis and where there is no metaxylem (centripetal wood) formed on its inner face. 
~ Exarch” describes any condition where all the primary metaxylem is formed on the 
‘ner or adaxial side of the protoxylem. 
