146 BOTANY [REPRODUCTIVE OKGA.NB. of the micropyle depends on the development of the nucleus, as well as on the thickness of the integuments. Where the integument is very thick and the nucleus small, the micropyle is a long canal, as in Hippuris ; but more usually the nucleus is large, and the integuments reduced to a few layers of cells, and in this case the micropyle is correspondingly reduced. The nucleus alters in the progress of growth so as to be prepared for the development of the embryo in its interior. A single cell of the nucleus near its centre enlarges greatly until it forms a hollow cavity surrounded by the smaller cells of the nucleus. This cavity is the embryo-sac (fig. 274, s), _and the protoplasmic contents have been termed the amnios. This embryo-sac increases in size, gradually supplanting the surrounding cellular tissue of the nucleus until it remains surrounded only by a thin layer of it ; or it may actually extend at the apex beyond it, as in Phaseolus and Alsine media ; or it may pass into the micropyle, as in Santalum. In Gymnosperms it usually remains deep in the nucleus and surrounded by a thick mass of cellular tissue (fig. 272). In Veronica, Euphrasia, and many Labiatse, the neck of the embryo-sac becomes elongated and swollen, and from it are developed certain vermiform or filamentous append ages, which are probably connected with the nutrition of the embryo. In some cases more than one embryo-sac is formed. This occurs in some gymnospermous plants, as the Yew, and it is also seen in Crucifer3. In the Mistleto several sacs are formed, but it is doubtful whether in this case several ovules have not coalesced. Usually only one becomes fully developed. When the embryo-sac has reached a certain stage of growth, a development of cellular tissue takes place within it by free cell formation. This occurs in all gymnospermous plants, and constitutes what is known as the endosperm (fig. 280, b). In augiospermous plants the endosperm is not formed until after fertilization, and it is rare to find any special development of cellular tissue within the embryo-sac before fertilization. But in some cases at the base of the embryo-sac a few cells are formed, which have been termed antipodal cells, and are frequently afterwards absorbed, or may be incorporated in the true endosperm when it is formed. The further processes of growth in gymnospermous ovules on the one hand, and angiospermous ovules on the other, differ so much that they must be noted separately. At the apex of the embryo-sac in gymnosperms, two or more cells of the endosperm enlarge so as to form what have been termed corpuscles (fig. 273). Each of these divides so as to form a large lower terminal or central cell, and an Fig. 273. Fig. 274. Fig. 275. IG. 273. Vertical section of the embryo-sac 6, and of part of the nucleus a, of the ovule of tho Wcymouth Pine (Hnus Strobus). At the micropylar end of the embryo-sac two cells, called corpuscles, d, have made their appearance. Each of these is at first separated from the inner surface of the micropylar end of the sac by a single cell, which afterwards divides into four, leaving a pas sage from the surface of the sac down to tho corpuscle. The pollen-grain r, on the summit of the nucleus, then sends down a tube which perforates the embryo-sac, and reaches the corpuscle through the intercellular canal. FIG. 274. Diagram of ovule of Polygonum, showing the nucleus n, and the embryo-sac *, containing a vesicle or germinal cell c, formed before impreg nation. This cell, after fertilization, develops the first cell e of the embryo. The dark shading at the base marks the chalaza, and the outer lines are the integument, the micropyle being opposite the clialaza. tio. 275. Campy! otropous or campylotropal ovule of Wallflower (Cficiranttius) showing the fonlcnlns /, which attaches the ovule to the placenta- j> the primuie, t the secundine, n, the nucleus, c/j, the chaluza. The ovule is curved upon itself, so that the foramen is near the f uniculus. upper neck of smaller cells (stiymatic cells), appearing as four when viewed from above, enclosing a canal. From the upper part of the central cell at the bottom of the neck, a small portion is separated the canal cell. It is from a portion of the central cell that, after fertilization, the embryo is formed. Those parts all have great physiological importance in connection with fertilization. In angiosperms from the protoplasmic mass at the apex of the embryo-sac by free cell formation two (rarely one) elongated ovoid cells are produced. These are the germinal vesicles (fig. 274). In some angiosperms as Crocus, Maize, &c., the vesicles placed side by side are both equally elongated ; to a con siderable extent their lower ends are rounded, and there it is that the nucleus is found. The upper end projects into the micropyle, and is marked by distinct striae longi tudinally. This portion has been distinguished as the filiform apparatus. From the germinal vesicles after impregnation the embryo is formed, only one vesicle developing. This filiform apparatus is considered as cor responding to the canal cell formed from the corpuscula in gymnosperms, and it seems to serve for conducting tho impregnating influence to the lower part of the central cell. In most angiosperms the germinal vesicles are placed obliquely over each other one attached to the apex of the sac, the other lower down, and there is no filiform apparatus. Of these two vesicles the lower one alone forms an embryo, the upper one performing the same function as the filiform apparatus of other angiosperms, and the canal cell of gymnosperms. The point where the integuments are united to the base of the nucleus is called the clialaza (fig. 278, cli). This is often coloured, is of a denser texture than the surrounding tissue, and is traversed by fibro-vascular bundles, which pass from the placenta to nourish the ovule. When the ovule is so developed that the point of union between the integuments and nucleus (the clialaza) is at tho hilum (next the placenta), and the micropyle is at the opposite extremity, there being a short f uniculus, the ovule is orthotropal, orthotropous, or atropous. This form is well seen in Polygonaceaa (fig. 271), Cistacea?, and most gymno sperms. In such an ovule a straight line drawn from tho hilum to the micropyle passes along the axis of the ovule. Where, by more rapid growth on one side than on the other, the nucleus, together with the integuments, is curved upon itself, so that the micropyle approaches the hilum, and ultimately is placed close to it, while the clialaza is at the hilum, the ovule is campylotropal or campylotropous (fig. 275) when tho portions onthe two sides of the line bisect ing the angle of curvature are unequal, or camptotropal when they are equal. Curved ovules are found in Leguminosa?, Cruciferaa, and Caryophyllacea3. Usually the opposite faces of the concavity of curvature of such ovules coalesce, and outwardly no indication of curvature is visible ; but in some cases there is no coalition, and the ovule is lecotropal, or Fig. 276. Fig. 277. Fig. 278. FIGS. 27fi, 277. Ovule of CheUdonium majm (cut longitudinally in fig. 277 to show the relation of its different parts). A. hilum or umbilicus; c, clialaza; /, funiculus or umbilical cord ; r, raphe ; n, nucleus ; ti, secundine ; te, primine ; ed, cndostome ; er, exostome. FIG. 278. Anatropous or anatropal ovule of Dandelion (Leontodon Taraxacum ), showing the coats of the ovule surrounding the nucleus n, which is inverted, so that its base ch, where the chala/a exists, is removed from the base or hiliim of the ovule A, while the foramen/ is near the base. The connection between tho base of the ovule and the base of the nucleus at s is kepi up by means of the raphc r. horse-shoe shaped. The inverted, anatropal, or anatropouu
ovule (figs. 27G, 277, 278) is the commonest form amongst