Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/157

Rh FERTILIZATION.] BOTANY 147 angiosperms. In this ovule the apex with the micropyle is turned towards the point of attachment of the funiculus to the placenta, the chalaza being situated at the opposite ex tremity; and the funiculus, which runs along the side usually next the placenta, coalesces with the ovule and constitutes the raphe (r), which often forms a ridge. The anatropal ovule arises from the placenta as a straight or only slightly curved cellular process, and as it grows, gradually becomes inverted, curving from the point of origin of the integuments. As the first integument (secundine) grows round it, the amount of inversion increases, and the funiculus becomes adherent to the side of the nucleus. Then if a primine be formed it covers all the free part of the ovule, but does not form on the side to which the raphe is adherent. Some anatropal ovules, however, seem to be formed in a different manner, the nucleus arising as a lateral bud from the apex of the funiculus, as in some Composites. These may be taken as the three types of ovule in the vegetable kingdom ; but there arc various intermediate forms, such as semi-anatropal, amphitropal, and heterotropal (transverse) ovules, where the funiculus is only, as it were, partially attached along one side, becoming free in the middle. The position of the ovule relative to the ovary varies. When there is a single ovule, with its axis vertical, it may be attached to the placenta at the base of the ovary (basal placenta), and it is then erect, as in Polygonacese and Composite ; or it may be inserted a little above the base, on a parietal placenta, with its apex upwards, and then is ascending, as in Parietaria. It may hang from an apicilar placenta at the summit of the ovary, its apex being directed downwards, and is inverted or pendulous, as in Hippuris vulgaris ; or from a parietal placenta near the summit, and then is suspended, as in Daphne Mezereum, ate XIII. Polygalaceje, and Euphorbiaccao. Sometimes a long funi culus arises from a basal placenta, reaches the summit of the ovary, and there bending over suspends the ovule, as in Armeria (Sea-pink) ; at other times the hilum appears to be in the middle, and the ovule becomes horizontal, peltate, or peritropous. When there arc two ovules in the same cell, they may be either collateral, that is, placed side by side (fig. 257), or the one may be erect and the other inverted, as in some species of Spiraaa and ^Esculus ; or they may be placed one above another, each directed similarly, as is the case in ovaries containing a moderate or definite number of ovules. Thus, in the ovary of Leguminous plants (fig. 256), the ovules o, are attached to the extended marginal placenta, one above the other, forming usually two parallel rows corresponding to each margin of the carpel. When the ovules are definite (i.e., are uniform, and can be counted), it is usual to find their attachment so constant as to afford good characters for classification. When the ovules are very numerous (indefinite}, while at the same time the placenta is not much developed, their position exhibits great variation, some being directed upwards, others downwards, others transversely ; and their form is altered by pressure into various polyhedral shapes. In such cases it frequently happens that some of the ovules are arrested in their development and become abortive. The homology of the ovule is by no means the same in all plants. In such cases as Polygonum and Piperacese, it represents the termination of the floral axis, and therefore is of the nature of a caulome. Again, in such plants as Primulacese and Composite, it is produced laterally upon the axis, and therefore represents a leaf, the integuments representing the lamina, and the funiculus the petiole, the nucleus being an outgrowth from them. In some instances of malformation a transformation into these parts actually takes place. In cases where they are produced on the margin of the carpellary leaves (the usual mode), the ovules represent lobes of a leaf, and in some cases of monstrosity, as in Delphinium datum, they appear as lobes of the carpellary leaf, whilst in Cupressus they are evidently outgrowths of the leaf. Further, the ovules in Orchidacese must be considered as mere trichomes, as they have no fibro-vascular bundles, and are developed from superficial cells of the placenta. When the pistil has reached a certain stage in growth it Fertilizj becomes ready for fertilization. Pollination having been tion. effected, and the pollen-grain having reached the stigma in angiosperms, the summit of the nucleus in gymnosperms, it is detained there, and the viscid secretion from the glands of the stigma in the former case, and the moisture from the ovule in the latter, induce the protrusion of the intine as a pollen-tube through the pores or points of perforation of the grain, many or few tubes being formed according to the number of pores. The pollen-tube or tubes pass down the canal (fig. 279), through the conducting tissue of the style when present, and reach the interior of the ovary in angiosperms, and then pass to the micropyle of the ovule, one pollen-tube going to each ovule. Sometimes the micropyle lies close to the base of the style, and then the pollen-tube enters it at once, but frequently it has to pass Fig. 279. Fio. 279. Pistil and pollen of Polygonum 1 Rtlgma, stfff, with pollen-grains p adherent to it, sending tubes tp down the conducting tissue of the style M ; the ovary o containing the ovule with its covering and central cellular mass or nucleus n, containing a rudimentary embryo-sac re, in which ultimately the embryo is developed. The base of the ovule attached to the placenta is marked by the chalaza ch. 2. Pollen-grainy, separated, with pollen-tube tp. FIG. 280. Vertical section of the ovule of the Scotch Fir (Pinus sylveitris) in May of the second year, showing the enlarged embryo-sac 6, full of cndospermal cells, and pollen-tubes c, penetrating the summit of the nucleus after the pollen has entered the large micropyle of the ovule. some distance into the ovary, being guided in its direc tion by various contrivances, as hairs, grooves, &c. In gymnosperms the pollen-grain resting on the apex of the nucleus sends out its pollen-tubes, which at once penetrate the nucleus of the ovule (fig. 280). In angiosperms when the pollen-tube reaches the micropyle it passes down into the canal, and this portion of it increases considerably in size. Where, as in Santalum album, Crocus, &amp;lt;fec., there is a filiform apparatus, the pollen-tube comes in contact with it. In most cases, however, it reaches the apex of the embryo-sac, sometimes slightly indenting it, as in Narcissus poeticus and Digitalis purpurea, or even perforating it, as in Canna. The granular protoplasmic matter in the pollen (fovilla) is then transmitted to the embryonal vesicle and fertilization is effected. Consequent upon this, after a longer or shorter period, those changes commence in the embryonal vesicle which result in the formation of the embryo plant, the ovule also undergoing changes which convert it into the seed, and fit it for a protective covering, and a store of nutri ment for the embryo. Nor are the effects of fertilization confined to the ovule; they extend to other parts of the plant. The ovary enlarges, and, with the seeds enclosed, constitutes