The large frond of Cycadites represented in fig.
A, part of frond; B, single pinna.
They consist of a so-called "frond" - a flattened green more or less oval structure which emits branches similar to itself from lateral pockets at or near the base.
Continuity between seeds and frond was also demonstrated in another species, A.
5, A), a characteristic Yorkshire fossil of Jurassic age, which in the form of the frond, bearing broad and relatively short pinnae, exhibits a striking agreement with the sterile portions of the fronds of Aneimia rotundifolia, a member of the fern family Schizaeaceae.
Finally, in the pinnae of the frond the centrifugal xylem may disappear, the protoxylem being now exarch in position and abutting on the phloem.
Hence palaeobotanists have found it necessary to adopt a purely :artificial system of classification, based on form and venation of the frond, in the absence of adequate data for a more natural, grouping.
Fertilis, from the Pottsville beds (Millstone Grit) of West Virginia, the rhomboidal seeds, flattened and winged like those of Cordaiteae, are borne terminally on the lateral pinnae of a frond, which elsewhere bears the characteristic cuneiform leaflets.
- Glossopteris frond, with portion enlarged to show the venation.
The leaves, borne on the regions between the false dichotomies, are those of Anomozamites minor, a type of Cycadean frond originally determined FIG.
- Frond of Williamsonia gigas.
E, Vertical section of frond of the complicated Siphoneous Green Alga Halimeda.
The substance of the frond is made up by a single much-branched tube, with interwoven branches.
This is the case in the Fucaceae, and in a very marked degree in the Laminariaceae in question, where the assimilative frond is borne at the end of an extremely long supporting and conducting stipe.
15, B); the foliage of Sphenopteris, one of the most extensive of Palaeozoic frond-genera, with many different types of fructification, resembled that of various species of Asplenium or Davallia.
6), recorded from several European localities, as well as from North America, Japan, China, Australia, India and Persia, affords an instance of a common type of bipinnate frond similar to Todites Williamsoni, which has been included in the Polypodiaceae; but such meagre evidence of the soral characters as we possess also points to a comparison with the recent fern Todea barbara.
One large specimen is figured by Heer from Lower Cretaceous rocks of Greenland, and by the side of the frond is shown a carpel with lateral ovules, as in the female flower of Cycas; but an examination of the type-specimen in the Copenhagen Museum led the present writer to regard this supposed carpel as valueless.
Professor Nathorst, as the result of a more recent examination of Heer's specimen, found that the segments of the frond are characterized by the presence of two parallel veins instead of a single midrib, with a row of stomata between them; for this type of Cycadean leaf he proposed the generic name Pseudocycas.