and spore-production, being in sharp contrast to the small slightly lobed brown leaves which act as stiff brackets (fig. 234, M) for collecting humus from which the roots absorb raw material. Similarly in _Platycerium_ the orbicular mantle-leaves differ widely from the long pendulous or erect fronds fashioned like the spreading antlers of an elk. In _Hemitelia capensis_, a South African Cyatheaceous species, the basal pinnae assume the form of finely divided leaves identified by earlier collectors as those of a parasitic _Trichomanes_ (fig. 235). In a letter written by W. H. Harvey in 1837 accompanying the specimen shown in fig. 235, he says, “Apropos of _Hemitelia_, be it known abroad that supposed parasitical _Trichomanes_ ... is not a parasite, but a part of the frond of _Hemitelia_.” The delicate reduced pinnae remain on the stem and form a cluster at the base of the fronds[713]. [Illustration: FIG. 233. _Polypodium Billardieri_ Br. (¼ nat. size.) Middle Island, New Zealand. From specimens in the Cambridge Herbarium.] [Illustration: FIG. 234. _Polypodium quercifolium._ (Much reduced: M, Mantle-leaves.)] In many species the sporophylls are distinguished from the sterile fronds by segments with little or no chlorophyllous tissue, as in _Onoclea struthiopteris_[714] in which, each year, the plant produces a funnel-shaped group of sterile leaves followed later in the season by a cluster of sporophylls; or, as in many other genera, the fertile leaves are distinguished also by longer petioles and thus serve as more efficient agents of spore-dissemination. In _Ceratopteris_ the narrow segments of the taller fertile leaves are in striking contrast to the broader pinnules of the submerged foliage leaves. Leaf-form is in many cases obviously the expression of environment; the xerophilous fern _Jamesonia_[715] from the treeless paramos of the Andes[716] is characterised by its minute leaflets with strong revolute margins and a thick felt of hairs on the lower surface; in others, xerophilous features take the form of a covering of overlapping scales (_Ceterach_), or a development of water-tissue as in the fleshy leaves of the Himalayan fern _Drymoglossum carnosum_. In the Bracken fern Boodle[717] has shown how the fronds may be classed as shade and sun leaves; the former are spreading and softer, while the latter are relatively smaller and of harder texture (fig. 236, _a_ and _b_). Even in one leaf six feet high, growing through a dense bush of gorse and bramble, the lower part was found to have the features of a shade leaf, while the uppermost exposed pinnae were xerophilous. [Illustration: FIG. 235. _Hemitelia capensis_ R. Brown. Nat. size. _a_, Pinna of normal frond. [From a specimen in the British Museum. M.S.]] [Illustration: FIG. 236_a_. _Pteris aquilina._ Part of leaf from greenhouse. (¼ nat. size.) After Boodle.] [Sidenote: PTERIS]