Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/158

BOTANY [REPRODUCTIVE ORGANS. the fruit, frequently incorporated with which are other parts of the flower, as receptacle, calyx, &c. In gymnosperms the pollen-tubes, having penetrated a certain distance down the tissue of the nucleus, are usually arrested in growth for a longer or shorter period, sometimes nearly a year. Subsequently growth recommences ; the tube advances to the apex of the embryo-sac, which it pierces, and reaches the mouth of the canal of the corpuscle. It either descends the canal or remains attached at the mouth. The fovilla is transmitted to the central cell, and fertilization is complete, the central cell giving rise to the embryo. _ In angio- sperms usually only one embryonal vesicle is fertilized, and one embryo is produced monoemlryony ; but in some plants where many embryonal vesicles are formed, as in Citrus and Scabiosa, several of them may be fertilized, and thus many embryos produced. polyembryony. Usually, however, only one develops, so as to be capable of ger mination or growth. In gymnosperms it is very com mon to have polyembryony, although produced in a dif- erent way, for each central cell of a corpuscle may produce four embryos, and as the central cell of more than one corpuscle may be fertilized a great many embryos may be formed. They do not, however, all come to maturity. 4. Female Organs of Phanerogams after Fertilization, a. The Fruit. After fertilization various changes take place in the parts of the flower. Those more immediately concerned in the process, the anther and stigma, rapidly wither and decay, while the filaments and style often remain for some time ; the floral envelopes become dry, the petals fall, and the sepals are either deciduous, or remain persistent in an altered form ; the ovary becomes enlarged, forming the pericarp ; and the ovules are developed as the seeds, con taining the embryo-plant. The term fruit is strictly applied to the mature pistil or ovary, with the seeds in its interior. But it often includes other parts of the flower, such as the bracts and floral envelopes. Thus the fruit of the Hazel and Oak consists of the ovary and bracts and calyx combined ; that of the Apple, Pear, and Gooseberry, of the ovary and calyx ; and that of the Pine-apple, of the ovaries and floral envelopes of several flowers combined. Such fruits are by some distinguished as pseudocarps. In popular language, the fruit includes all those parts which exhibit a striking change as the result of fertilization. In general, the fruit is not ripened unless fertilization has been effected : but cases occur in which the fruit swells, and becomes to all appearance perfect, while no seeds are produced. Thus, there are seedless Oranges, Grapes, and Pine-Apples. When the ovules are unfertilized, it is common to find that the ovary withers and does not come to maturity ; but in the case of Bananas, Plantains, and Bread fruit, the non-development of seeds seems to lead to a larger growth, and a greater succulence of fruit. The fruit, like the ovary, may be formed of a single carpel, or of several. It may have one cell or cavity, being unilocular ; or many, multilocular, &c. The num ber and nature of the divisions depend on the number of carpels, and the extent to which their edges are folded inwards. The appearances presented by the ovary do not always remain permanent in the fruit. Great changes are observed to take place, not merely as regards the increased size of the ovary, its softening and harden ing, but also in its internal structure, owing to the sup pression, additional formation, or enlargement of parts. Thus, in the Ash (fig. 281) an ovary with two cells, each containing an ovule attached to a central placenta, is changed into a unilocular fruit with one seed ; one ovule, I, becoming abortive, while the other, g, gradually enlarging until the septum is pushed to one side, unites with the walls of the cell, and the placenta appears to be parietal. In the Oak and Hazel, an ovary with three cells, and two ovules in each, changes into a one-celled fruit with one seed. In the Coco-nut, a trilocular and triovular ovary is changed into a one-celled, one-seeded fruit. This abortion may depend on the pressure caused by the development of certain ovules, or it may proceed from mm fertilization of all the ovules and consequent non-enlarge ment of the carpels. Again, by the growth of the placenta, or the folding inwards of parts of the carpels, divisions occur in the fruit which did not exist in the ovary. In Pretrea zanzibarica, a one-celled ovary is changed into a four-celled fruit by the extension of the placenta. In Cathartocarpus Fistula a one- celled ovary is changed into a fruit, having each of its seeds in a separate cell, in consequence of spurious dissepiments (jphragmata) being produced in a horizontal manner, from the inner wall of the ovary. In Linum, by the folding inwards of the Fig. 282. FIG. 281. Samara or Samaroid fruit of Frc.riinis oxypliylhi. Kntirc, with its wing a. 2. Lower portion cut transversely, to show that it consists of two loculaments; one of which, I, is abortive, and is reduced to a very small cavity, while the other is much enlarged, and filled with a seed g. FIG. 282. Drupe of the Cherry (Ccrasvs), cut vertically, showing the skin, or epicarp ep, the flesh or mcsocarp me, and the stone, putamen, or endocarp en, enclosing the seed g, with the embryo. back of the carpels a five-celled ovary becomes a ten- celled fruit. In Astragalus, the folding inwards of the dorsal suture converts a one-celled ovary into a two- celled fruit ; and in Oxytropis the folding of the ventral suture gives rise to a similar change in the fruit. The development of cellular or pulpy matter, and the en largement of parts not forming whorls of the flower, frequently alter the appearance of the fruit, and render it difficult to discover its formation. In the Gooseberry, Grape, Guava, Tomato, and Pomegranate, the seeds nestle in pulp, formed apparently by the placentas. In the Orange, the pulpy matter surrounding the seeds is formed by succulent cells, which are produced from the inner partitioned lining of the pericarp. In the Strawberry the receptacle becomes succulent, and bears the carpels on its convex surface (fig. 1 54) ; in the Eose there is a fleshy hollow torus or disk, which bears the carpels on its concave surface (fig. 155). In the Juniper the scaly bracts grow up round the seeds and become succulent, and in the Fig (fig. 150) the receptacle becomes succulent and encloses an inflorescence. The pistil, in its simplest state, consists of a carpel or folded leaf, with ovules at its margin ; and the same struc ture will be found in the fruit, where the pericarp represents the carpcllary leaf, and the seeds correspond to the ovules. The pericarp consists usually of three layers, the external, or epicarp (fig. 282, ep), corresponding to the lower epi dermis of the leaf ; the middle, or inesocarp, me, representing the parenchyma of the leaf ; and the internal, or cndocarp, en, equivalent to the upper epidermis of the leaf, or the

epithelium of the ovary. These layers are well seen in such