Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/114

 104 F E R N S several free veins arise, bearing sori at their tips, as in the section Campyloneuron of Polypodium. 6. Where distinct main veins run from the midrib to the edge of a frond or its divisions, with copious small irregular areols3 between them, as in the section Pleuridium of Poly- podium arid Gymnopteris of Acrostichum. 7. Where the whole space between midrib and margin is filled up with copious small irregular areolae, as in the section Phymatodes of Polypodium and Chrysodium of Acrostichum. Many of the large genera, as characterized by the shape and position of the sori, contain groups which show several of these different plans of veining ; Polypodium and Acrostichum show them nearly all. One school of system- atists, represented in England by J. Smith and Moore, regard a decided difference in veining as a character im portant enough in itself to mark a genus ; and if this plan be followed the number of genera is enormously increased. Our estimate of 50 for the genera of Polypodiacece is a large one, if genera founded upon fructification be under stood ; but Moore, using venation hi addition, reckons the genera at 143, and John Smith at 194. From this different scale upon which genera have been planned arises the large number of different generic names which have often been used for the same plants, so that the synonymy of ferns has become extremely complicated. The sporangia of ferns are morphologically trichomes or modified hairs. They originate from epidermal cells, and in the mature state consist of stalked capsules, the walls of which are one cell thick. A ring of cells differently placed in different genera is called the annulus^ and by its unequal contraction in drying effects the disruption of the capsule and the dispersion of the spores (fig. 5). In some genera Fig. 5. Fig. 6. FIG. 5. Mature sporangium of the Male Fern (yephrodiumFHijc-mas). p, stalk; a, annulus; s, spores discharged from ruptured cavity. FIG. 6. Leaf of Polypodium of the section Goniophlebium, bearing soi i on its back. FIG. 7. Soms of Aspiditim trifoliatum. The annulate sporangia are covered by an orbicular indusium attached by its centre. the same object is effected by a group of cells instsad of a ring. The separate clusters, of few or many individual sporangia, are called sori (fig. G), and the membranous excrescence of the epidermis which covers a sorus is called an indusium or involucre (fig. 7). The orders of Filices are founded upon differences in the structure of the individual sporangium. Genera of the same order depend upon differences in the shape and posi tion of the sori and shape and position of the indusium. In a few genera, such as Osmunda, Anemia, and Schizcea, the parenchyma of the lamina is entirely absorbed in the fertile parts of the frond, and the sporangia form distinct panicles or spikes. In Acrostichum the sporangia cover the whole of the under surface of the frond, both veins and inter spaces. In many genera, such as Pteris, Vittaria, and Lind- saya, the sori run in a line along the very edge of the frond, connecting the tips of numerous veinlets. In Blechnum they form a similar line close to the midrib. The commonest type of all is when they form round dots on the veins of the back of the frond, as in Polypodium^ Aspidium, Woodsia, and Cyalhea. A type only less common than the last is where they run in lines along the veins of the back of the frond, as in Asplenium, Scolopendrium, and Gymnogramme. In Lygodium the sporangia form spikes on the edge of the leafy segments, each sporangium inclosed in a little pocket. In Dicksonia, Hymenophyllum, and Trichomanes, the sori form round clusters at the tip of a vein on the edge of the lamina. Characters taken from the indusium separate the genera of Polypcdiacex into four tolerably well-marked groups. In a large series of genera, including Polypodium with not less than 400 species, Acrostichnm, and Gymno gramme, there is no indusium at all. In another series, including Pteris, Pellcea, Cheilanthcs, and Ilypolepis, there is what is called a spurious indusium, which consists simply of the edge of the frond modified in texture and more or less completely recurved, so as to protect the sorus in an early stage. The true indusia are distinct scales, and are of two kinds, those called inferior, which originate beneath the sorus, and may be distinctly cup-shaped, as in Woodsia and Cyathea, or may be one-sided, as in Cystopteris and Davallia ; and those called superior, as in J^ephrodium and Aspidium) which are extended over the sorus. As has been already indicated, all the genera of Polypodiacea 1, which include in the aggregate not less than 2000 species, agree with one another in the structure of the sporangia, so that the great majority of existing forms of Filices are very closely allied. The classification of ferns is based at present upon characters entirely drawn from the sporophore generation, and takes no account of the oophore. It is un doubtedly to a great extent artificial, but until the develop ment of the prothallium has been carefully worked out in all the different orders, no more satisfactory or more natural arrangement than the following appears attainable. 1. Gleickeiiiacccz. Sporangia sessile, splitting vertical! y, furnished with a complete horizontal ring. Sori composed of very few spo rangia ; receptacle not elevated. Fronds with very distinct dicho- tomous branching. Genera 2 ; species 30. 2. Hymcnophyllaccw. Sporangia sessile, splitting vertically, fur nished with a complete horizontal ring. Sori composed of numerous sporangia, inserted on a long filiform receptacle. Fronds of filmy texture, with pinnate branching. Genera 2 ; species 150-200. 3. Cyatheacece. Sporangia nearly sessile, splitting transversely, furnished with a usually incomplete nearly vertical but rather oblique ring. Receptacle prominent, barrel-shaped. Tree-ferns. Genera 3 ; species 150. 4. Pol ypodiacea. Sporangia stalked, splitting transversely, fur nished with a usually incomplete vertical ring. Receptacle not prominent. Genera 50 ; species 2000. 5. Osmwulacece. Sporangia stalked, splitting vertically, furnished with only a faint horizontal bar instead of a ring. Genera 2 ; species 10-12. 6. Sckizccacccc. Sporangia sessile, splitting vertically, crowned by a complete small annular horizontal ring. Genera 5 ; species CO. The oophore, or sexual generation of ferns, not having been studied to any great extent from the comparative point of view, can only be described in general terms. It is a membranous structure furnished with root hairs, and the cells of which contain chlorophyll ; it is therefore capable of obtaining its nourishment independently, and in many respects has a striking resemblance to the simpler Hepatii-n. 1 . When the spores (for example) of any Polypodiaceous fern are sown, the outer covering bursts, and the inner coat pro trudes as a filament, which soon divides and continues to grow into a short row of cells. Cell division next begins to take place laterally as well as longitudinally, and a small cellular plate is formed (fig. 8), which gradually attains a kidney-shaped outline, the growing point being situate in the notch. From the under side are developed the root hairs, antheridia and archegonia.