27° BOTANICAL GAZETTE [APRIL 
the exarch group, but one which lies close to the cordaitean alli- 
ance, has given rise to the Ginkgoales, the Coniferales, and prob- 
ably the angiosperms. Centripetal wood persists here only in 
Ginkgo, and in the leaves of such ancient conifers as Prepinus. 
Endarchy is apparently elsewhere universal, although our knowl- 
edge of conditions in the earlier forms is very slight. Ginkgo and 
the more primitive conifers possess a double foliar bundle, and all 
members of both groups have parallel veined leaves. The double 
bundle also appears in seedlings of certain angiosperms, but this 
series has otherwise departed very widely from its palaeozoic 
ancestry. ; 
A third line from our ancient stock gave rise to the cycad-like 
Bennettitales which flourished in the Cretaceous. The foliar trace 
here was a single bundle—either the primitive single strand, perhaps 
comparable to that of Medullosa, or a fusion of two—which ies 
veloped in the petiole and blade into a series of typically exarc 
bundles with centrifugal secondary wood. The reproductive 
organs were very specialized, and the group has probably given rise 
to no living family of plants. : 
The modern Cycadales perhaps constitute a fourth line = 
the palaeozoic plexus which, however, lies very close to that 0 
the Bennettitales. They have often been considered as derived es 
forms related to the Lyginodendreae, but the exarch structure © 
the leaf bundles and the construction of the seed, which is much more 
comparable to that of Medullosa than to that of Lyin 
make it very improbable that the latter genus has given rise to co 
modern family. The cycads are also separated from the Benne 
tales by the possession of a double leaf trace and a simple fis 
ductive system. The parallel venation of the leaf suggests ' a 
Cordaitales, and, as we have remarked above, Nilssonia show 
an intermediate condition between the two types of leaf. Se 
It seems’ probable that the group arose in the late pees 
iferous from cycad-ferns possessing a double exarch leaf trace a” 
petiolar bundle, a type of leaf intermediate between the cre 
fern frond and the parallel veined cordaitean condition, and a see 
resembling that of Medullosa. 
. ries, 
There are thus apparently two main groups in the exarch series, 
