generally absorbed also, by which the pollen-cells are set free in the cells of the anther. Sometimes the membrane of the special mother-cells is not com pletely absorbed, in which case the pollencells of the mother-cell are more or less connected, and form a compound body consisting of four pollen-cells; or if the membranes of two or more united mothercells are also incompletely absorbed, we may have a mass consisting of eight pollen-cells, or of some multiple of four, as in many species of Acacia. In the Onagraceæ, again, the pollen-cells remain FIG. 144.—Pollinia, £, loosely connected by long viscid threads, which appear to be derived from the imperfect solution of the mother-cells; while of Orchis with their caudicles, c, and the retinacula, r, r, at the base. in the Orchidacea the pollen-cells cohere in a remarkable degree, and form pollen-masses which are commonly of a waxy nature, to which the name of pollinia has been given (Fig. 144, p). In the Asclepiadaceæ also, somewhat similar masses occur (Fig. 145, p, and b); but in the latter, the whole surface of each pollen-mass is invested by a special cellular covering. By a careful examination of these pollinia we find that they are formed of compound masses agglutinated together, and, when separated, each of these masses is found to consist of four pollen-cells. In the pollinia of the Orchidaceæ we also find other peculiarities; thus each is prolonged downward in the form of a stalk called the caudicle (Fig. 144, c), which adheres commonly at the period of dehiscence to one or two little glandular masses called retinacula (Figs. 146, a, and 144, FIG. 145.-Pistil of a species of Asclepias, with the pollinia,, adhering to the stigma, s. b. Pollen - masses sepa rated. 116 CELLS WALLS OF POLLEN-CELLS. r, r), which are placed on the upper surface of a little projection called the rostellum situated at the base of the anther. Structure of the Pollen.—We FIG. 146.-Upper part of the flower of an Orchis, showing the pollinia adhering to the stigma by the retinacula, a. shall treat of this subject under three heads, viz.: 1. Wall; 2. Contents; 3. Form and Size. 1. Walls or Coats of the Pollen-cell. When mature the wall of the pollen-cell generally consists of two membranes; an internal or intine, and an external or extine. But in rare cases there is but one membrane, which is of a similar nature to the intine. The intine is the first formed layer, and appears to be of the same nature and appearance in all pollen-cells. It is usually smooth, very delicate and transparent, and composed of pure cellulose. It is generally applied so as to form a complete lining to the extine, except perhaps in those cases where the latter presents various processes, when it is probable that the intine does not extend into them in the mature pollen. The extine is a hard, thick, resisting layer forming a kind of cuticle over the intine or proper cell-coat; and, while the latter usually presents a similar appearance in the pollen of different plants, the extine is liable to great variation; thus it is sometimes smooth, and in other cases marked with little granular processes or spiny protuberances (Fig. 147) or reticulations. The nature of these markings is always the same for the pollen of any particular species of plant, but varies much in that of different plants. The color of pollen-cells, which also resides in the extine, is in by far the majority of cases yellow, but various other col FIG. 147-Pollen ro CONTENTS OF POLLEN-CELLS. 117 ors are also occasionally found; thus the pollen-cells are red in species of Verbascum, blue in some species of Epilobium, black in the Tulip, rarely green, and occasionally of a whitish tint. FIG. 148.-Elliptical pollen of Milkwort (Polygala). e. Extine. f.Slits. Besides the various markings just described as existing on the extine, we find also either pores or slits (Fig. 148, ƒ), or both pores and slits, and which vary in number and arrangement in different plants. In the greater number of Monocotyledons there is but one slit; while three is a common number in Dicotyledons. Sometimes there are six, rarely four, still more rarely two, and in some cases we find twelve or more slits. The pores, like the slits, also vary as to their number. Thus we commonly find one in Monocotyledons, as in the Grasses; and three in Dicotyledons. again, the pores are very numerous, in which case they are either irregularly distributed, or arranged in a more or less regular manner. The pores, also, may be either simple, or provided with little lid-like processes; these are pushed off by corresponding projections of the intine when the pollen bursts, or when it falls upon the stigma for the purpose of fertilizing the ovules. Sometimes The pollen of all Angiospermous plants is a simple cell as above described, but in Gymnospermous plants it contains other small cells, which adhere to the inside of its internal membrane close to the point where the external membrane presents a slit. These minute cells are termed daughter-cells. 2. Contents of Pollen-cells.-The matter contained within the pollen-cell is called the fovilla. This consists of a dense coarsely-granular protoplasm, in which are suspended very small starch-granules, and what appear to be oil-globules. As the pollen-cell approaches to maturity the fovilla becomes more concentrated, and contains less fluid matter and more granules. The fovilla is without doubt the essential part of the pollen-cell, but the office it performs will be explained hereafter. 3. Forms and Sizes of Pollen-cells.-Pollen-cells are found of various forms. The most common forms appear to be the spherical; sometimes they are polyhedral, or triangular with the angles rounded and enlarged (trigonal), as in plants of the order Onagraceæ, or cubical, or cylindrical. It should also be noticed that the form of the pollen is materially influenced according as it is dry or moist. In size, pollen-cells vary from about to Too of an inch in diameter; their size, however, like their form, is liable to vary according as they are examined in a dry state or in water. II. The Disk.-The term disk is variously applied by botanists; but in this work it is understood to include all bodies of whatever form which are situated on the thalamus between the calyx and gynoecium, or upon or in connection with either of these organs, but which can not be properly referred to them, and as it is most commonly placed between the androecium and gynoecium, it is best treated of in this place. The disk is developed in a variety of forms; as a fleshy ring surrounding the base of the pistil; as a dark red cup-shaped expansion covering nearly the whole of the pistil except the stigmas; as a sort of waxy lining to the tube of the calyx; and in Umbelliferous plants it exists as a more or less evident swelling on the top of the ovaries adhering to the styles; and has been termed the stylopodium. In other cases the disk is reduced to little separate glandular bodies, as in Cruciferous plants; or to scales, or to various petaloid expansions, as in the Columbine. When the disk is situated under the ovary, it is termed hypogynous; when it is attached to the calyx, it is perigynous, or when on the summit of the ovary it is epigynous. III. The Gynocium or Pistil.-The gynocium, or pistil as it is frequently called, is the central or terminal organ of the flower; and it consists of one or more modified leaves called carpels, which are either distinct from each other or combined into one body. When there is but one carpel, as in Leguminous plants, the pistil is said to be simple; when there is more than one, whether distinct from each other or combined into one body, it is described as compound. The Carpel.-Each carpel consists, first, of a hollow inferior part arising from the thalamus, called the ovary, containing in its interior one or more little somewhat roundish or oval bodies called ovules, and which are attached to a projection on the walls termed the placenta ; and, second, of a stigma or space of variable size, composed of loose parenchymatous tissue without epidermis, which is either placed directly on the ovary, when it is said to be sessile, or it is elevated on a stalk prolonged from the ovary, called the style. The only essential parts of the carpel, therefore, are the ovary and stigma, the style being no more necessary to it than the filament is to the stamen. The terms ovary, style, and stigma are applied in precisely the same sense when speaking of a compound pistil, in which the parts are completely united, as with the simple carpel. The simple ovary (page 122) has two sutures, one of which corresponds to the union of the margins of the lamina of the carpellary leaf out of which it is formed, and which is turned toward the axis of the plant; and another, which corresponds to the midrib of the lamina, is directed toward the floral envelopes or to the circumference of the flower; the former is called the ventral suture, the latter the dorsal. (See also page 138.) Nature of the Carpel.—That the carpel is analogous to the leaf is proved in various ways, but we shall here only allude to the proofs of its nature which are afforded by the appearance it sometimes presents in double or cultivated flowers. Thus, in a double flower of the Cherry |