Order 1. Equisetaceae.—Horse-tails. Homosporous.

3. Lycopodinae.-Club Mosses. Stems elongated, dichotomously branched, either forked or forming a sympodium, with leaves, in many cases greatly reduced, or submerged, shortened, and tuberous with awl-shaped leaves. Sporangia arising singly in the form of firm-walled capsules either from the stem in the leaf-axils, or from the leaf-base. Tapetal cells persistent.

Order 1. Lycopodiaceae.-Club Mosses. Homosporous.
Order 2. Selaginellaceae.-Heterosporous.

Order 3. Isoetaceae.-Heterosporous.

There are also various fossil groups, some of which are included in the above divisions, while some form independent classes.

CLASS I

Filicinae (Ferns) (124)

The

The great majority of existing Pteridophytes belong to the Ferns, taking the group in a wide sense. Two sub-classes are distinguished according to the structure of the sporangia. Eusporangiate Ferns are characterised by sporangia, the thick wall of which consists of a number of layers of cells. They open by a transverse split. The Leptosporangiate Ferns on the other hand have sporangia which when mature have their wall formed of one layer of cells. Stipules, which are found at the base of the frond in the former group, are wanting in the Leptosporangiatae. Differences also exist in the prothallus and in the structure of the sexual organs.

While in earlier geological periods the Eusporangiatae were abundantly represented, they now include only two families, each with a few genera. They appear to represent the more ancient type of Ferns and to stand nearest to the forms from which both Filicinae and Lycopodinae have been derived. Along with them even in palaeozoic times we have the Leptosporangiatae, from which the Hydropterideae have branched off as a small group of aquatic or marsh-growing Ferns. In the Hydropterideae only among Ferns the spores are differentiated into microspores and macrospores.

Sub-Class I. Eusporangiatae

Order 1. Ophioglossaceae (125)

European examples of this order are afforded by Ophioglossum vulgatum, Adder's Tongue (Fig. 392, B) and Botrychium, Moonwort (Fig. 392, A). Both have a short stem, from which only a single leaf unfolds each year. The leaves in both cases are provided with leaf-sheaths. In Ophioglossum the leaf is tongue-shaped, in Botrychium it is pinnate. These leaves bear on their upper side a fertile

segment arising near the upper end of the leaf-stalk. This fertile segment in Ophioglossum is simple and cylindrical, with the sporangia sunk in two rows; in Botrychium it is pinnately branched in the upper part, and thickly beset on the inner side with large nearly spherical sporangia.

Our knowledge of the peculiar monoecious prothalli of the Ophioglossaceae is largely due to BRUCHMANN; they are long-lived, subterranean, saprophytic,

FIG. 392.-A, Botrychium iunaria. Sporophyte. nat. size.) B, Ophtoglossum vulgatum. Sporophyte showing the bud for the succeeding year. (nat. size.) C, Ophioglossum vulgatum. Prothallus. an, antheridia; ar, archegonia; k, young plant with the first root; ad, adventitious branch; h, fungal hyphae. (x 15; after BRUCHMANN.)

tuberous bodies without chlorophyll but inhabited by a mycorhizal fungus. In Ophioglossum (Fig. 392, C) they are cylindrical and radially symmetrical, simple or branched; in Botrychium they are oval or heart-shaped and dorsi ventral. The antheridia (Fig. 393) and archegonia (Fig. 394) are sunk in the tissue of the prothallus. The antheridium encloses a large spherical mass of spermatozoid mother-cells which are set free when mature by the swelling of the contents and the breaking down of one of the central cells of the wall. The spermatozoids have a spirally wound body and numerous cilia; a small vesicle is adherent to the spermatozoid (Fig. 393, E). The antheridia originate from single superficial cells

(Fig. 393, 4-C), as do also the archegonia (Fig. 394, A-C). The slightly projecting neck of the latter opens after the neck-canal-cell has swollen and disintegrated; the oosphere (o) remains in the sunken venter. In many species the embryo leads a subterranean existence for several years. The primary root is first formed and soon projects from the archegonium (Fig. 392, C, k); later the first leaf and the apical cell of the stem are differentiated. In some species of Botrychium the embryo forms an elongated multicellular suspensor at the end of which the proper embryonic mass is formed (126). In this an agreement with the Lycopodinae is evident (cf. Fig. 412 and Fig. 417).

This order includes a number of stately tropical Ferns with thickened tuberous stems and usually very large leaves provided with two stipules at the base. The sporangia are situated in groups (sori) on the under surface of the leaves, and are either free (Angiopteris), or united to form an oval capsule-like body, the chambers of which are the sporangia. The prothallium in contrast to that of the Ophioglossaceae is a green, heart-shaped thallus, resembling that of a Liverwort and growing on the surface of the soil. It is sometimes dichotomously branched. The sexual organs resemble those of the preceding order but are developed on the lower surface of the prothallus. An endophytic fungus occurs in the cells of the prothallus (127).

Sub-Class II. Leptosporangiatae

Order 1. Filices

The Filices, or Ferns, in the narrower sense of the word, comprise a large number of genera with numerous species, being widely distributed in all parts of the world. They attain their highest development in the tropics. The TreeFerns (Cyathea, Alsophila, etc.), which include the largest representatives of the order, occur in tropical countries, and characterise the special family of the Cyatheaceae. The stem of a Tree-Fern (Fig. 395) is woody and unbranched: it

bears at the apex a rosette of pinnately compound leaves or fronds, which are produced in succession from the terminal bud, and leave, when dead, a large leaf scar on the trunk. The stem is attached to the soil by means of numerous adventitious roots. The majority of Ferns, however, are herbaceous, and possess a creeping rhizome, terminating usually in a rosette of pinnate or deeply divided leaves. Such a habit and growth are illustrated by the common Fern Aspidium filix

3

FIG. 395.-Alsophila crinita. A Tree-Fern growing in Ceylon (Reduced.)

mas, the rhizome of which is official (Fig. 396). When young, the leaves of this Fern are coiled at the tips (Fig. 396, 1, a), a peculiarity common to the Ferns as a whole, and to the Water-Ferns. Unlike the leaves of Phanerogams, those of the Ferns continue to grow at the apex until their full size is attained. The leaves of the common Polypodium vulgare are pinnate, and spring singly from the upper side of the creeping branched rhizome. In other cases the leaves may be simple and undivided, as in the Hart's-Tongue Fern, Scolopendrium vulgare (Fig. 397). In the tropics many herbaceous Ferns grow as epiphytes on forest trees.

Peculiar brownish scales (paleae, ramenta), often fringed and consisting of a

FIG. 396.-Aspidium filix mas. (nat. size.) A, Sorus in vertical section (x 20, after KNY); B, pinna with young sori still covered by the indusia; (, somewhat older sori with withered indusia. (Slightly magnified.) OFFICIAL.

single layer of cells, invest the stems, petioles, and sometimes also the leaves of most Ferns.