Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/113

FERNS 103 cushions and mattresses ; and the clathrate flat paleae, dull brown in colour and permeated by a distinct lattice-work of thicker cells, which occur in Yittaria, Antropkyian, and some species of Asplenium. By their abundance, position, size, shape, and texture, the palese often furnish great help in characterizing species. /Stipes. Although generally distinguished by this spe cial name, the stipes is simply the petiole of the leaf. It is very rare to find a fern in which the blade of the leaf does not possess a distinct stalk. In about two- thirds of the Ft/ices, including not only thjse with erect, but many with creeping stems, the stipes is con tinuous with the stem, without any articulation at the point of junction; and when the old fronds die, they do not become disjointed, after the fashion of the leaves of a deciduous tree. In about one-third of the Filices, on the contrary, there is a distinct joint at the point of junction between the stipes and the stem, and when the fronds fade, they fall away from the stem spontaneously, leaving a clean scar. In the classification proposed by John Smith, the Polypodiacece are subdivided by this character. The ferns in which the leaf-stalk is continuous with the stem, as in Asplenium, Nephrodium, and Pier is, he calls Desmobrya. Those in which the old fronds become disarticulated at the base, as in Oleandra, Platycerium, and Polypodium vulgare, he calls Eremobrya ; and he states, as the result of his experience as curator for 40 years of the Kew collection, that the Desmobrya may be more easily reproduced from spores, but that the Eremobrya show greater tenacity of life, and that in the importation of ferns from abroad the Eremobrya generally arrive in good condition, whilst the Desmobrya, especially such as have slender stems, often perish in the journey. Fronds. In Filices we get a more varied range of leaf- form than in any other order in the vegetable kingdom, and some of the most beautiful and complicated types that exist. In size the fronds vary from under an inch, as in some of the minute Hymenophyllce, to a length and breadth of from ten to twenty feet in the great Cyatheas and Dicksonias. To take examples of the leading types from amongst familiar British species, we have simple fronds in ttcolopendrium vulgare, simply pinnate fronds in Asplenium Trichomanes andLomaria Spicant, bipinnate in Nephrodium montanum and Polypodium Phcgopteris, tripinnate or tripinuatifid in Cystopteris fragilis and Polypodium Dryopteri*, and decompound in Asplenium Adiantum-nigrum and Ptcris aquilina. The primary divisions of a compound frond are called pinna}, the secondary divisions pinnules. In a deltoid frond, as in Cystopteris montana or Ptcris pal- mata, the pinna? grow regularly smaller from the lower to the upper, and the lower pinnoti have the pinnules of the under side exaggerated. A decrescent frond, as in Nephrodium conterminum or Asplenium Trichomanes, has the pinnae largest at the centre of the frond, and gradually smaller both towards its top and bottom. There is no regular relation between the frond and its fructification. Species with fronds similar in texture, cutting, and veining are often found, when the fructification is examined, to stand widely apart in systematic position. The growth of the fronds is very slow. The whole of the fronds of a rosette of Nephrodium Fil uc-mas have been in course of formation two years before the lamina begins to unroll. The develop ment is basifugal, and often the fronds continue growing at the top a long time after they are fully unfolded and producing fructification at the base. This may be seen iu llymenophyllum sericeum and Gymnogramme elongata ; iu Lygodium the leaf-stalk resembles a twining stem, growing for a long time. In many cases the fronds are of two kinds, developing from the first in a conspicuously different man ner, one kind remaining permanently barren, and the other alone producing fructification. We have instances of this in Lomaria, tipicant and Cryptogramme crispa, and di morphism is shown still more conspicuously in Plat y cerium, in which the barren fronds are uncut and spread out flat over the surface on which the plant grows, and the fertile ones are much larger, and stand erect, and are forked, from which it takes its common name of elk s-horn fern. There is another striking instance of dimorphism in the section Drynaria of the genus Polypodium, in which the barren fronds are brown in colour, rigid in texture, small in size, and lobed like the leaf of the common oak, and the fertile ones are several feet long, with elongated segments like those of Polypodium vulgare. In some ferns the fronds have a habit of taking root at the tip, and thus producing new plants. This is well shown in the walking-leaf of North America (ticolopendrium rhizophyllum], and amongst tropi cal ferns in Asplenium rldzopliorum and Acrostichum Jlagclliferum. Some species develop copious adventitious buds, from, which the plant may be reproduced, sometimes scattered over the surface of the frond, as in Asplenium viviparum and A. buWiferum, sometimes in the axils of the pinna?, as in Asplenium prolifervm and Nephrodium cicutarium. Venation. In the arrangement of the veins of the fronds of Filices there is an extraordinary amount of variety. The different types, and the extent to which they prevail, and the use that may be made in the systematic arrangement of ferns of the characters they furnish, were carefully worked out by Presl and John Smith. The two principal leading types are veins that, after once branching, do not again unite, and veins that, on the contrary, after branching, join one another so as to inclose areoke like the meshes of a net. Ferns that exhibit the first plan are said to be free-veined ; the other kind are said to have anasto mosing venation. The free-veined ferns form far more than half of the order. In the whole frond or its ultimate divi sions there may be a midrib alone, as in Munogramme graminea or llymenophyllum tunbridgense ; or the veining may be simply pinnate, that is to say, a midrib with a series of parallel simple branches on each side, as in Nephrodium patens or N. prolixum ; or there may be a re gular midrib from the base to the top of the frond or seg ment, with the side-branches branched again, a very com mon type shown in Ptcris aquilina and Nephrodium Filix- mas ; or there may be no regular midrib, but the veins arranged like the rays of a fan, as in Adiantum Capillus- Yeneris and Asplenium cuneatum. Whether the veins are produced to the margin, or end abruptly without reaching it, as is common in the free-veined, and still more so in the net-veined species of Polypodium, is a point to be noted. Of anastomosing venation the following are the principal types : 1. Where the veins are free nearly to the edge, but con nected by an arch just within it, as in the section Tham- nopteris of Asplenium and Olfersia of Acrostichum. 2. Where the veins are arranged iu parallel pinnate groups, and the opposite veins of contiguous groups join at the tip, as in the section Eunephrodium of Nephrodium and Goniopteris of Polypodium. 3. Where the veins of contiguous segments are copiously pinnate and free, but the midribs are connected by a single arch and the base, as in the section Campteria of Pteris and Pleocnemia of Polypodium. 4. Where the veins unite in regular hexagonal meshes the sori borne at the tip cf a single central vein which these inclose, as io the section GoniopJdtbium of Poly . 5. Where distinct main veins run from the midrib to the edge of a frond or its divisions, and these are connected by a series of parallel arches, from the upper side of which