Fig. 235. Hemitelia capensis R. Brown. Nat. size. a, Pinna of normal frond.
[From a specimen in the British Museum. M.S.]
Fig. 236a. Pteris aquilina.
Part of leaf from greenhouse. (¼ nat. size.) After Boodle.
PTERIS
The resemblance between some of the filmy Hymenophyllaceae and thalloid Liverworts[718] is worthy of mention as one of the many possible pitfalls to be avoided by the palaeobotanical student. The long linear fronds of such genera as Vittaria and Monogramme might well be identified in a fossil state as the leaves of a grass-like Monocotyledon, or compared with the foliage of Isoetes or Pilularia. The resemblance of some fern leaves with reticulate venation to those of Dicotyledons has led astray experienced palaeobotanists; it is not only the anastomosing venation in the leaves of several ferns that simulates dicotyledonous foliage, but the compound leaves of many dicotyledons, e.g. Paullinia thalictrifolia (Sapindaceae) and species of Umbelliferae, may easily be mistaken for fronds of ferns.
Fig. 236b. Pteris aquilina.
Leaf from the same plant grown out of doors. (¼ nat. size.)
After Boodle.
RECENT FERNS
The dichotomously lobed lamina of some Schizaeas, e.g. S. dichotoma and S. elegans ([fig. 222]), bears a close resemblance to the leaves of Baiera or Ginkgo[719]. The original description by Kunze[720] of the South African Cycad Stangeria paradoxa as a Polypodiaceous fern illustrates the difficulty, or indeed impossibility, of distinguishing between a sterile simply pinnate fern frond and the foliage of some Cycads. The deeply divided segments of Cycas Micholitzii[721] simulate the dichotomously branched pinnae of Lygodium dichotomum, and the leaves of Aneimia rotundifolia ([fig. 223]) and other species are almost identical in form with the Jurassic species Otozamites Beani, a member of the Cycadophyta.
There are certain facts in regard to the geographical distribution of ferns to which attention should be directed. Mr Baker in his paper on fern distribution writes: “With the precision of an hygrometer, an increase in the fern-vegetation marks the wooded humid regions[722].” If in a collection of fossil plants we find a preponderance of ferns we are tempted to assume the existence of such conditions as are favourable to the luxuriant development of ferns at the present day. On the other hand, we must bear in mind the wonderful plasticity of many recent species and the fact that xerophilous ferns are by no means unknown in present-day floras.