A spore on germination produces a structure which, compared with its immediate parent, is relatively very small, and bears no resemblance to it in form or texture. It is called the prothallium; it is green and membranous, and attached to the surface of the ground by root hairs in ferns and horse tails; in adder's tongues and club mosses it is tuberous and subterranean, while in Rhizοcarpece and Selaginella it always remains more or less included within the cavity of the parent spore. The function of the prothallium is entirely reproductive; it develops sexual organs of two kinds, archegonia and antheridia, either upon the same or different prothallia. Hence, in contradistinction to the sporophore—the function of which in this group of plants is purely vegetative—the prothallial generation is termed the oophore.

History.— The older botanists, starting from the study of flowering plants, endeavoured to recognize in all other cases the same characteristic points of structure. They did not doubt therefore, arguing from analogy, that ferns were provided with true seeds. Gerarde (1597), remarking theappearance of young plants of NephroHum dilatatum in the neighbourhood of old ones, attributed this to the dissemination of seeds, “for I believe all plants have seeds in themselves to produce their kindes.” No one, however, at this time could say anything definite about the “seeds” of ferns, and they came to be regarded as highly mysterious. It was supposed that they were of an invisible sort, and by a transference of properties it came to be believed that the possessor of fern seed would be invisible too. Shakespeare makes Chamberlaine say, “We have the receit of fern-seed, we walke invisible.” Ray ridicules Tragus for spreading linen clothe upon the ground to catch the seed of ferns on the eve of midsummer night, when, as Ray properly observed, it would be indeed astonishing if any fell, seeing that it is not mature till the autumn.

Columna in 1648 compared the fern frond to butcher’s broom, and identified the fructification in the one case with the flowers in the other. Caesius came to his help, and, having examined the sori of a polypody, “telescopii adjumento,” detected what he supposed to be seeds, but which were probably only the spore cases. W. Cole appears to have been the first to microscopically observe the spores themselves (1669), and Ray himself described the hygroscopic movements of the spore-cases which assist the dispersion of the spores. Morison (1715) seems to have been the first to put the matter to the test of actual experience. He sowed the spores of hart’s tongue, and in due course, without, however, distinguishing clearly the two stages, raised plants from them. This important observation fell completely into oblivion, and when in 1789 Dr Lindsay, a Scotch physician settled in Jamaica, responded to a request of Sir Joseph Banks for fern plants from that island by suggesting that spores would be much more convenient to send, the latter was quite unprepared for the suggestion, which he treated as a great discovery. Dr Lindsay accordingly wrote a paper on the subject (which was published in the Linnean Transactions a few years afterwards), in which he figured the prothallial stages of Polypodium lycopodioides. The spores of ferns were still, however, regarded as equivalent to seeds, t.c., as the result of a process of fertilization similar to that which precedes the development of seeds in flowering plants. A variety of attempts were made to see in hairs or glands upon the young fronds, &c., something that would do duty for an anther. As late as 1832 De Candolle maintained the analogy of the prothallium to a cotyledon. In 1844 Nägeli discovered the antherozoids, motile bodies which are set free from the antheridium, and which perform in fems the function of pollen grains. All previous observers had tried to find fertilizing bodies on the vegetative generation, the sporophore, which produces only spores. In 1844 the final step was reached when Suminski found upon the prothallia the archegonia, flaskshaped bodies containing a central cell (oosphere) which, when fertilized by the antherozoids, develops into a sporebearing plant similar to that from which the cycle started. In 1850 Mettenius discovered the details of the reproduction in Isoetes. Since that time the researches upon the Pteridophyta have been very numerous, and are to be found quoted in Sachs’s Lehrbuch.

The whole group of the Pteridophyta may be classified as follows :—

Pteridophyta. —Conuophyta with two distinct stages in the life-cycle. Sporophore with high vegetative differentiation. Oophore inconspicuous and destitute of vascular tissue.

Class I. Filicince.— Leaves highly developed. Sporangia numerous on the fertile leaves.

Sub-Class 1. Filices. — Leaves without stipular appendages. Sporangia epidermal, containing spores of one kind developed in each from a single primary mother-cell.

Sub-Class 2. Stipulate:.— Leaves with stipule-like appendages. Sporangia containing spores of one kind developed in each from many endogenous primary mother-cells.

Sub-Class 3. Rhiaocarpeoe. — Spores of two kinds.

Class IL Equisetinae. — Leaves rudimentary. Sporangia 5-10 on the fertile leaves.

Class III. Lycopodinae. — Leaves small (except Isoetes'}, simple. Sporangia solitary.

Sub-Class 1. Lycopodiaceae. — Spores of one kind.

Sub-Class 2. Ligulatae. — Spores of two kinds.

I. Filicinae. —1. The Filices or ferns proper deserve consideration at greater length, both on account of their numerical preponderance over the other groups, and the popular favour which deservedly attaches to their great beauty and variety of form.

Commencing with the vegetative generation or sporophore, the following remarks briefly touch upon the most essential points in their organization.

In general habit, although with a certain characteristic appearance which, to the experienced eye, is an almost unfailing distinctive character, ferns are as various in stature and aspect as flowering plants. One genus, Cerαtopteris, is remarkable for its aquatic habit. The terrestrial forms vary from the Hymenophyllaceae, which have the small size and delicate texture of mosses, to the larger forms vzhich attain the size of shrubs, while some belonging to the tropics and the southern hemisphere havo a palm-like habit, and are called tree-ferns. Cyathea medullaris reaches 80 feet in New Zealand, and Alsophila australis even greater heights in Australia. The stem creeps on or beneath the surface of the ground as in the common bracken, or climbs up rocks and the trunks of trees {Davalliα canariensis). In the latter case it is usually densely coated with imbricated membranous scales. In stems of this type the internodes or spaces between the insertion of successive leaves (fronds) is considerable, and the growing point is often far in advance of the youngest frond. In a large number of herbaceous ferns, of which the common male and lady fems are good examples, the stem is short and erect or slightly inclined (fig. 1). The fronds are developed spirally, and in close contiguity without obvious internodes; they form, viewed from above, a large rosette with the growing point in the centre. Between the ordinary herbaceous ferns with short erect stems and the tree-ferns there is no hard and sharp line of demarcation. Most of the ferns of decidedly arborescent habit belong to the sub-order Cyatheaceae, which has a sporangium and receptacle a little different from that of the typical Polypodiaceae, but in many of the genera of Poly podiaceae, especially Dicksonia, there are species that assume a more or less decidedly arborescent habit Nearly all Filices are perennial, but we have an instance of annual duration in Gymnogramme leptophylla. Growing Point.— The apex of the stem, whatever the dimensions of the mature plant, is always occupied by a single apical cell, and the whole growth is entirely dependent upon the repeated segmentation of this cell. In form it is either wedge-shaped with a transverse section like that of a bi-convex lens, or it has the form of a three-sided pyramid with a convex base forming the outer face. In the former case the segments are produced from the cell alternately right and left, and the stem, which is gradually built up by the repeated subdivision of the segments so formed, is always distinctly bilateral, and produces its fronds in two rectilinear series. In the latter case the segments are derived in spiral succession from the three interior faces of the apical cell, and the leaves, which are developed from the masses of tissue to which these segments give rise, are gradually displaced from the simple triseriate arrangement due to the segmentation of the apical cell into more complicated forms of phyllotaxis.

Branching is effected by the partition of the growing point; the single stem therefore bifurcates. This scarcely ever happens in ferns with erect stems, such as Nephrodium Filix-mas, or in tree-ferns. New shoots are, however, sometimes formed from the bases of the leaf-stalks, just as in some species they are normally produced from the fronds themselves. When the terminal point is injured, lateral shoots are often formed in this way. A specimen of Dick sonia antarctica has been described with as many as twenty-nine crowns, due doubtless to such a cause.

Structure of the Stem. — The repeated subdivision of the cells of the growing point gives rise to a mass of tissue, at first uniform in character, but which is subsequently differentiated. Particular tracts of cells are modified in the process of growth to form the fibro-vascular bundles. These are more or less flattened plates irregularly elliptical in transverse section. As in all the Pterιdophyta they are closed, and incapable, when once formed, of further increase in diameter. They consist of a mass of “xylem” surrounded by “phloem.” The xylem consiste, in addition to a few spiral vessels at the foci of the elliptical tranverse section, of scalariform vessels—cells of comparatively large diameter, with oblique ends, and with their walls intermittently thickened by transverse projecting ridges (fig. 2). Amongst the vessels are oblong cells containing starch in winter (woodparenchyma). The phloem contains sieve-tubes, and external to these thick-walled bast fibres. The whole fibro-vascular bundle is bounded first by one or two layers of oblong starch-containing cells (phloem-sheath), and external to these by a single layer of narrower cells, often thickened externally and darker in colour—the vascular bundle sheath. In very slender stems the fibro-vasculαr bundles form a central axis tflymenophyllaceae), but when the stems attain any size they are arranged in an anastomosing cylindrical network (fig. 3), in which a mesh corresponds to the base of a frond. This cylinderseparates the remaining or fundamental tissues of the stem into two portions, an internal central medulla and an externaΓcortex. This is shown in fig. 4; the dark lines, however, do not belong to the bundles which correspond to the light spaces enclosed by them; the dark lines themselves consist of a mass of thickened elongated cells termed “sclerenchyma,” which form protecting sheaths to the fascicular tissue. The medulla and cortex are in the main formed of parenchymatous tissue, often loaded with starch. In Cyathea medullaris this starchy pith was used by the Maoris as an article of food. The medulla is not, however, entirely parenchymatous; it is usually permeated by a number of anastomosing filiform bundles (usually also invested with a sclerenchymatous sheath), which eventually pass into the leaves through the meshes in the fibro-vascular cylinder. The greater part of the fibro-vascular tissue of the leaves is, however, continuous with the lower portion of the margins of the meshes, which runs out into bundles arranged round the outer circumference of the leaf stalk, and surrounding therefore the bundles which pass through the meshes from the medulla of the stem. In some cases, however, these medullary bundles contract an adhesion to the margins of the meshes as they pass through them. The surface of the stem, and also of the leaf-stalks,

13 usually strengthened by the sclerenchymatous development of the superficial portions of the fundamental tissue. This coats the surface with a layer which would be impervious to air were it not interrupted by spaces filled with soft parenchyma, by which the internal tissues of the stem are brought into relation with the atmosphere. In Pteris aquilina, the rhizome or horizontal subterranean stem is marked laterally by two lines where the colourless cortical parenchyma is not covered in, and. in many tree-ferns the bases of the petioles have their sclerenchymatous investment perforated by pits filled in the living plant by a soft-celled tissue which speedily becomes pulverulent.

The roots of ferns are always slender and wiry. They are continually formed during the growth of the stem in acropetal succession. In tree-ferns they consequently accumulate on the lower portion as they grow downward, giving it a large apparent diameter. In creeping species they at once attach themselves. They are cylindrical in all cases, scarcely exceeding one-eighth of an inch in diameter, dark-brown approaching black in colour, and clothed with root-hairs.

In ferns the occurrence of a special form of “trichomes” is noticeable in the flat multicellular membranous scales, to which the name of “paleae” is given. They are found in the greatest abundance on the stem and lower part of the stipes, but they often extend to the rachides of the frond, and sometimes to the foliar surfaces, especially in young fronds. There is no definite correlation in the character of these paleae with other points of structure. The most characteristic are the long thread-like bright-brown paleae of EiLdicksonia, which are so abundant that those of Dicksonia Culcita are exported in quantity from Madeira to stuff 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 Vittama, Antroρhyum, and some species of Asplénium. By their abundance, position, size, shape, and texture, the paleae often furnish great help in characterizing species.

Stipes. —Although generally distinguished by this special 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 Filices, including not only those with erect, but many with creeping stems, the stipes is continuous 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 Polypodiaceae are subdivided by this character. The ferns in which the leaf-stalk is continuous with the stem, as in Asplénium, Nephrodium, and Pteris, he calls Desmobrya. Those in which the old fronds become disarticulated at the base, as in Oleαndra, Platycerium, and Polypodium vulgäre, 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.— Iu Filices we get a more varied range of leafform 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 Hymenophyllae, 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 Scolopendrium vulgare, simply pinnate fronds in Asplénium Trichomoenes aηdLomaria Spicant, bipinnate in Nephrodium montanum and Polypodium Phegopteris, tripinnate or tripinnatifid in Cystoptems fragilis and Polypodium Dryopteris, and tecorn^oandinAsplenuimAdiantum-nigrum and Pteris aquilina. The primary divisions of a compound frond are called pinnae, the secondary divisions pinnules. In a deltoid frond, as in Cystopteris montana or Pteris palmata, the pinnae grow regularly smaller from the lower to the upper, and the lower pinnae have the pinnules of the under side exaggerated. A decrescent frond, as in Nephrodiτιm conterminum or Asplénium 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 Filix-mas have been in course of formation two years before the lamina begins to unroll. The development 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 Hymenophyllum sericeum and Gymnogramme elongata; iu Lygodium the leaf-stalk resembles a twining stem, growing for a long time. Iu many cases the fronds are of two kinds, developing from the first in a conspicuously different manner, one kind remaining permanently barren, and the other alone producing fructification. Wo have instances of this in Lomaria Spicant and Cryptogramme crispa, and dimorphism is shown still more conspicuously in Platycerium, 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 Dryηaria 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 {Scolopendrium rhizophyllum), and amongst tropical ferns in Asplénium rhizoplωrwn and Acrostichum flagelliferum. Some species develop copious adventitious buds, from which the plant may be reproduced, sometimes scattered over the surface of the frond, as in Asplénium viviparum and A. bulbiferum, sometimes in the axils of the pinnae, as in Asplénium proliferwm 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 fems 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 areolae 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 anastomosing venation. The free-veined ferns form far more than half of the order. In the whole frond or its ultimate divisions there may be a midrib alone, as in Monogramme graminea or Hymenoρhyllum tuηbridgense; 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 regular midrib from the base to the top of the frond or segment, with the side-branches branched again, a very common type shown in Pteris aquilina and Nephrodium...