83. The embryo.—Immediately after the en trance of the spiral filaments into the cavity of the archegonium, the cells which immediately surround it multiply rapidly, in consequence of which the cushion-like projection of the in ferior surface of the prothallium increases in size. At the same time the germ-cell is trans formed into an irregularly egg-shaped body, which consists of minute cellules, and may be considered as the priniary axis of the future fern. It originates in the same manner as the rudiment of the fruit of the Mosses and Hepaticm, and elongates by repeated divisions of a terminal cell by septa, inclined alter nately in opposite directions. It consequently presents but one growing point, which is di rected, not towards the orifice of the archego nium, but, on the contrary, towards the centre of the cushion-like mass, by the cells of which it is surrounded. Soon after, however, there appears on the side of the egg-shaped embryo, which is directed towards the notch in the anterior margin of the prothallium, a second growing projection of its surface. This projection, at first conical, becomes, as it enlarges, compressed from above downwards. No soon er is this the case, than it bursts through the superficial cellular la) er of the prothallium, at a point which is invariably a little anterior to the base of the archegonium—between it and the angle of the notch. It now assumes the form of a symmetrical leaf-like organ, and begins to project beyond the notch of the prothallium. The further development con sists in the appearance in the axil of this primordial leaf, of a new axis, the permanent stem of the young plant. From this axis all the succeeding leaves take their origin, each diverging from its immediate predecessor at an angle of 60°.
84. Sporangia and spores.—At a point of the surface of the frond, which always cor responds to the termination of a vascular bundle, a lacuna is formed under the epider mal layer, by the separation of that structure from the subjacent tissue. The floor of this cavity consists of a pavement of tessellar cells, some of which grow out into nipple shaped projections. In each of these, the projecting portion is separated from the rest by a horizontal septum, which is soon fol lowed by several others superior and parallel to it. The last-formed terminal cell now en larges, and assumes a globular form, and is converted by a process similar to that to be described below in the rudimentary sporan gium of Equisetum, into a central mass of nucleated cells, with grumous contents (parent cells), enclosed in a capsule formed of a single layer of others, which are tabular. In each parent cell, the central nucleus afterwards disappears, and is replaced by four others. This is followed by the division of the pri mordial sac into four portions, around each of which a cellulose membrane is formed. This membrane becomes the epispore; asecond (endospore), which is distinguished by its greater delicacy, being subsequently formed within it.* 85. Equisetacece.—The history of the deve lopment of the Equisetacm corresponds in most respects with that of the Ferns.
86. .First period.t —Germination of the spore. — The spore of Equisetum consists, in its ripe condition, of a delicate, colourless internal vesicle, which is surrounded by a rnore or less resistant granular membrane, and contains a central nucleus, and a yellowish grumous fluid, in which swim oil and chlorophylle granules. The first change observed in germination con sists in the division of the nucleus into two, and the subsequent formation of a septum between the two corresponding halves of the spore-cell. Of these halves, the larger con tains nearly all the chlorophylle, and is de veloped to the stem ; the smaller,the con tents of which are almost colourless, is the commence ment of the root. The prothallium, which results from repeated cell-division of the larger half, is an irregularly riband-shaped expan sion, growing and branching repeatedly at the extremity furthest from its point of origin, and consisting of large, delicate-walled cells, containing much chlorophylle. One of the branches is usually observed to be larger than the rest, and it is upon it that the reproductive organs are formed.
87. Antheridium.—The rudimentary antheri dium of Equisetum consists, like that of pre ceding families, of a papilla, composed of four conjoined vertical piles of cells, each pair slightly overlapping the pair preceding it. In each of the cells constituting this rudiment a tangential wall is formed, dividing it into an inner three-sided, and an outer tabular cell. The inner cells, which form a central oval mass, are soon observed to be filled with finely granular mucus : the tabular cells, on the con trary, contain chlorophylle, and fortn the wall of the future antheridium. The further de velopment of the central mass corresponds entirely with what has been described in other families. The antherozoa are larger in Equi setum than in any other known example. They originate by the deposition of a gela tinous linear thickening, in the form of an imperfect ring, parallel to the plane surfaces of the discoid vesicles in which they are enclosed. When fully formed, they resemble, in almost every respect, those of the Ferns. The ridia of the Equisetacew are placed, not upon superimposed upon each other, which com bine to form a circular wall round a central cavity, which contains the germ, and is the termination of the quadrilateral canal. On the transformation of the germ-cell into the embryo, observations are as yet wanting.
89. Spores and sporangia.—The organs upon which the spore-cases are supported are ar ranged in whorls round the upper part of the fruit-bearing stein. They seem to be modifications of the ordinary stem-leaves, on which account they have received the name of sporophylla. In its earliest condition, the sporophyllum is a cellular projection of the surface ; but, as it advances towards maturity, it assumes the form of a hexagonal disc the inferior surface, but along each margin of the principal branch of the prothallium.