We now proceed to consider the. embryogeny of (1) gymnospermous and (2) angio spermous phanerogams. In the gymnospermous or naked-seeded flowering-plants, such as the comferce and cycadacw. impregnation is effected by direct contact between the pollen and the ovule, there being no true ovary bearing a stigma. The process is thus summarized by Balfour: " In gymnospermous plants, there are stamens containing pollen, and ovules supported on cones or altered branches, and in them the pollen enters the large micropyle of the ovule without the intervention of stigma or style. ' When the pollen reaches the nucleus of the naked ovule, it remains long dormant, and after many weeks and months, sends out a tube which reaches the embryo sac, and impreg nates a corpuscle. One of the cells of the corpuscle then takes an active function, and develops the embryo with the suspensor in the midst of endospermal cells."—Op. cit., p. GOO. In the angiospermous phitherogams, when the pollen-tube has tra versed the tissue of the style, and reached the ovule, it proceeds through the foramen, or micropyle, so as to 'come in contact with the embryo sac; and consequent on this is the development of the cellular embryo. There is, however, much dispute as to what now occurs. " Schleiden thinks that the end of the pollen tube introverts the embryo sac, and in some cases perforates it, and that it becomes the first cell in the embryo. Most physi ologists, however, agree in thinking that Sebleiden was mis taken in regard to the extremity of the pollen-tube, and they believe that the embryo is formed from a distinct cell pre viously existing Ili the embryo sac. In some instances, the pollen-tube indents the embryo sac, at other times it perfor ates it, and conies into actual contact with a cell contained to the sac. In the embryo sac there are produced, before impreg nation, certain cells, often three, which are called germinal vesicles, only one of which in general is impregnated by the pollinic fluid, which transudes through the membrane of the pollen-tube and the walls of the embryo sac and vesicles. After • impregnation, the vesicle divides by a transverse septum into two parts, the upper portion forming a confervoid partitioned filament or suspensor, and the lower becomes filled with cells, constituting the rudimentary embryo. The suspensor is at , niched to the part which forms the radicle of the embryo, and at the opposite end, one or two cotyledons are produced, inclos ing the fresh bud or plumule. An embryo is usually produced in each ovule (monembryonomy); but when more than one germinal vesicle is impreg nated, there is a of embryos (polyembryonomy). When the pollen of one species is applied to the pistil of another species, we occasionally find seeds produced which rive rise to individuals intermediate between the two parents: these individuals are called hybrids or mules, and are rarely fertile. A plant has, however, a preference: for the pollen of its own species, and hence hybrids are rare in nature."—Balfour, op, p. 600. A reference to the preceding figure of a section of part of the ovule of a species of Speedwell, will elucidate the above summary: it shows the pollen-tube a, just as it reaches the embryo sac which contains the rudimentary embryo, d, attached to the sac by its suspensor, b, and endospermal cells, c, at the lower part of the sac. The suspensor is sometimes of considerable length, and as much as three, or even five times the length of the whole seed. Its attachment to the radicular end of the embryo
is shown in fig. 8. In monocotyledons, a single sheathing cotyledon is developed; in dicotyledons, two opposite leaves; and after their formation, the apex produces the terminal bud or plumule. The embryo is thus suspended in an inverted position in the seed.
It is impossible to enter into any general description of the organs or process of reproduction in cryptogamic plants. In this great division of the vegetable kingdom, the organs of reproduction are in general obscure, and consist usually of cellular sacs of two kinds—one being called antheridia, containing phytowa or spermatozoids, represent ing the stamens, or the male; and the other being called piztillidict or arehegonia, and representing the pistil, or the female. In the fully developed state of the plant, the antheridia disappear, while the pistillidia are transformed into cellular sacs containing gevninating bodies known as spores (q.v.), which are considered as being formed by a process of reproduction, and as being analogous to cellular embryos. These spores are developed in mother-cells, the contents 'of which often divide into four, such mother cells being called sporidia. With regard to the antheridia and the pistillidia in the dif ferent orders of cryptogamic plants, Dr Balfour observes that in ferns they are supposed to exist in a pro-thallus or cellular expansion produced by the spore when it germinates. A cell of the pistillimir(the ovular body) afterward gives rise to the spore-bariug leaves (the fronds). After impregnation, the archegonial cells give rise to a sporingiferous frond. The spores are contained in sporangia, with or without an elastic ring, devel oped on the back, on the side, or at the base of the leaves. In mosses, these organs are seen at certain stages of the plant's growth, and they are either on the same or on different plants. After impregnation, the archegonial cell gives rise to a stalked theca or sporangium with its spores. In liverworts, they are usually on different parts of the plant, and as frequently in the substance or on the under surface of disk-shaped cellular stalked expansions. Here the impregnated cell gives rise to the fruit or capsules. In lich ens,the existence of these organs has not been alreadyestablished; and the fructification consists of them or asci, containing 4, 8, 12, or 16 sporidia (or cells containing spores) in their interior. These thecae are usually united together so as to form a cup-like mass of fructification. When mature, the sporidia or thecm burst, and disdharge the spores. The fungi, antheridia, and pistillidia are obscure, and the organs of reproduction are spores which are either naked or are contained in thec?. In algm, antheridia and pistil lidia have often been detected; but in some of them, certain cells, in the same or sepa rate filaments, seem to possess the property of producing spores by a process of conju gation or union; and in the lowest forms the cells undergo division into new individ uals.
Besides the above-noticed modes of propagation, cryptogamic plants are also propa gated by buds or gemmm, which are either attached to the leaves or fronds, or are con tained in peculiar cup-shaped bodies. See Carpenter's General and Comparative Anatomy, and Balfour's Class-book of Botany, from which we have borrowed freely.