tetrahadrally arranged special spore-cells. In the megasporangium one of the special spore-cells grows and forms the megaspore, that becomes enveloped in an epispore formed from the mucilage resulting from the disintegration of the remaining sixty-three mother-cells and the cells forming the tapetum. In the microsporangium the whole of the sixtyfour special mother-cells develop into microspores. The megaspores have an apical umbo or papilla, below which the prothallus is formed and finally exposed by the rupture of the epispore. The prothallus never becomes free from the megaspore, and is generally destitute of chlorophyll, or contains a small portion. The archegonia consist of a ventral portion and a short neck, and are produced in smaller numbers than in preceding groups, and in some genera only one is formed. The prothallus formed by the microspore is exceedingly rudimentary, being usually reduced to a single cell, the antheridia are also rudimentary as compared with those of preceding orders. The antherozoids are spirally coiled, and furnished at the broader end with vibratile cilia and a vesicle containing starch grains. The fertilized oosphere becomes clothed with a cell-wall, and then by repeated division forms a foot, cotyledon and plumule, before becoming free from the megaspore.

It will have been observed that in the present group the male and female organs respectively originate from independent structures-the microsporangium and macrosporangium—the number of archegonia have been considerably reduced, and the oophore or sexual generation reduced to a minimum, the male prothallus to that of a single cell, the female one to a small group of cells that do not become free from the megaspore, hence the great bulk of the plant, including all the vegetative portion, the wall of the

sporocarp, with its contained megaspores or microspores, belonging to the asexual generation or sporophore.

Fossil Vascular Cryptogams.-Several of the groups were more numerous and highly differentiated in early geological periods than at the present day. The earliest remains occur in the Upper Silurian, and the maximum of development appears to have been reached during the Devonian and Carboniferous ages, where, along with Gymnosperms, they constituted the most highly organized of existing vegetation, the Angiosperms occurring first in the Permian formation. The Rhizocarpea are scantily represented in the earlier formations, but fossil forms nearly allied to the Selaginellacea occur in great abundance in Paleozoic times, especially during the Carboniferous period, where the species of Lepidodendron and Sigillaria assumed the dimensions of modern forest trees, having stems eighty or ninety feet high and two feet or more in diameter, covered with scars of various patterns, corresponding to the point of attachment of leaves. The fructification is heterosporous, the sporangia occupying the bases of leaves collected into dense, terminal, cone-like spikes. The fossils once known by the name of Stigmaria have been proved to be the roots of species of Sigillaria. Ferns are especially abundant during the Carboniferous period, all the species being referable to existing types. The Equisetacea are represented in Carboniferous strata, more especially by species of Calamites, having the habit of Equisetum, but much larger than any existing species.

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CHAPTER VIII.

PHANEROGAMIA.

Gymnospermeæ-Angiospermeæ.

As previously stated, the group known as Phanerogams or Flowering plants must not be considered as an entirely new starting-point, altogether independent of Cryptogams, but rather as a continuation of the latter group, characterized by certain morphological and physiological features, but at the same time quite as closely allied to the higher Cryptogams, or even more so, as the divisions of the latter are to each other.

In Phanerogams the male or fertilizing element is never a naked protoplasmic body or antherozoid possessed of spontaneous movement that reaches the oosphere by the agency of water; but on the other hand its protoplasm is permanently enclosed in a cell-wall, the whole being known as a pollen grain, fertilization being effected by the protrusion. of the intine or inner wall of the pollen grain in the form of a slender filament called the pollen-tube, which either directly or indirectly comes in contact with the oosphere, or ovule as it is frequently called, and along which the nucleus of the pollen grain passes and enters the oosphere, thus effecting the act of fecundation. Owing to the absence of spontaneous movement on the part of the pollen, the latter is either produced in close proximity to the female organ on which it is shed, or it may be carried by the wind or by

insects. In the highest Cryptogams the asexually-produced spores always become free from the sporophore before they produce the sexual organs; but in Phanerogams fertilization takes place upon the sporophore, the structure that becomes free being the highly-differentiated embryo still surrounded by a protective covering belonging to the sporophore.

Gymnospermeæ.

The Gymnosperms, represented by the firs, pines, yews, cedars, cycads, etc., are most closely allied to the vascular Cryptogams. The pollen grain is homologous with the microspore of the heterosporous Vascular Cryptogams, and the pollen-sac of the anther with the microsporangium. In all Gymnosperms the pollen grain consists of two or more cells, one of which is very much larger than the rest, and produces the pollen-tube that effects fertilization. This large cell corresponds to the fertile portion of the microspore that gives origin to antherozoids, while the smaller cells are homologous with the rudimentary prothallus of the microspore. In the female reproductive organ of Gymnosperms, we find within certain integuments a structure called the nucellus, which is the homologue of the megasporangium in Vascular Cryptogams. The nucellus encloses the embryo-sac, the homologue of the megaspore, which in turn contains several secondary embryo-sacs or corpuscula, that correspond to the central cell of the archegonium in Vascular Cryptogams; the archegonium itself is arrested, but certain rudimentary structures are present that correspond to the neck of the archegonium. Several oospheres are usually formed, although only one becomes fertilized. The endosperm is formed as in Selaginella, before the

oospheres are fertilized (Fig. 38). After fertilization, the oosphere always forms a suspensor, or pro-embryo, a structure first indicated in Selaginella. Fertilization is always direct, the pollen grain coming directly in contact with the ovule, that is seated on an open carpellary bract. The vascular bundles are collateral and open. True vessels are rare.

Angiospermæ.

In the present large division, that includes the great majority of Phanerogams, the pollen grain is generally unicellular, the single cell being fertile and forming pollen tubes, hence the last trace of the male prothallus has disappeared. In the female organ the embryonic vesicle is also formed directly in the embryo-sac, hence the oophyte or sexual generation is reduced to the pollen grain and the oosphere. The endosperm is formed after the fertilization of the oosphere. Fertilization is always indirect, the ovules always being enclosed within one or more carpellary leaves, that constitute the pericarp or "fruit." The fibro-vascular bundles are collateral, and the xylem contains true vessels in considerable quantity. The two subdivisions, Monocotyledones and Dicotyledones, are characterized as follows.

Monocotyledones: Embryo, with one cotyledon or seedFibro-vascular bundles closed.

leaf.

Dicotyledones: Embryo, with two opposite cotyledons. Fibro-vascular bundles open.

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