Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/155

Rh OVULE.] BOTANY 145 lateral, as in Asimina, or on both sides, bilateral, as in Plantago. Occasionally, as in Tasmannia, it is prolonged along the inner surface of the style. In Iris it is situated on a cleft on the back of the petaloid divisions of the style (fig. 268). Some stigmata, as those of the Mimulus, present sensitive flattened laminae, which close when touched. The stigma presents various forms. It may be globular, as in Mirabilis Jalapa ; orbicular, as in Arbutus Andrachne ; umbrella-like, as in Sarracenia, where, how ever, the proper stigmatic surface is beneath the angles of the large expansion of the apex of the style ; ovoid, as in Fuchsia ; hemispherical ; polyhedral ; radiating, as in the Poppy (fig. 269), where the true stigmatic rays are attached to a sort of peltate or shield-like body, which may represent depressed or flattened styles ; cucullate, i.e., covered by a hood, in Calabar Bean, where it is situ ated on the apex of a declinate style, bearded (hairy) on its concave surface The lobes of a stigma may be flat and pointed, as in Mimulus and Bignonia, fleshy and blunt, smooth or granular, or they may be feathery, as in many Grasses (fig. 213). In Orchidaceae the stigma is situated on the anterior surface of the column formed by the union of the styles and filaments, the point where it occurs being called (jynizus. In Asclepiadacese the stigmas are united to the face of the anthers, and along with them form a solid mass (fig. 220). Transformations of the pistil are of frequent occur rence, and depend generally on abortion of a certain number of carpels, and on adhesions of various kinds. In the apocarpous pistils of Aconite, Nigella, Larkspur, and Pieony, we find on the uame plant pistils composed of two, three, four, five, and six carpels. In some of the Brambles, all the carpels except one have been observed to disappear, thus making the fruit resemble that of the Plum. In the case of Leguminous plants there is usually only a single carpel, although the flower is pentamerous; this state has been traced to abortion of car pels, and the view is confirmed by finding plants in the same natural order with more than one carpel. Pistils of a succu lent nature, such as those of the Sloe and Bird-cherry, some times assume the form of a pod, like that of the Pea. Occa sionally stamens are changed into carpels, and at other times the carpels are transformed into stamens, and bear pollen. The ovule is the body attached to the placenta, and destined to become the seed. Ovules are most usually produced on the margins of the carpellary leaves, but are also formed over the whole surface of the leaf, as in Cupressus. In other instances they rise from the floral axis itself, either as terminal buds, as in Polygonaceae and Pipcraceee, or as lateral buds, as in Primulaceas and Compositce. The ovule is usually contained in an ovary, and all plants in which the ovule is so enclosed are termed anyiospermous ; but in Conifene and Cycadacere it is generally considered as having no proper ovarian covering, and is called naked, these orders being denominated^nwio- xpermous The gymnospermal view is not adopted by all botanists, some maintaining that there is a true ovarian covering In Cycas the altered leaf, upon the margin of which the ovule is produced, and the peltate scales, from which they are pendulous in Zamia, are regarded by all botanists as carpellary leaves. But in the Coniferse great discussion has arisen regarding the morphology of parts in many genera ; some considering the scales at the base of the scaly bracts of the cone as a placental process growing from the bract, which is thus a carpellary leaf opened out and bearing a sessile ovule, the whole cone representing a single flower ; while others, again, regard the scale as an ovular integument, and the ovule as being destitute of ovary, the outer scales being bracts, and the cone therefore being an inflorescence. The carpellary leaves are sometimes united in such a way as to leave an opening at the apex of the pistil, so that the ovules are exposed or seminude, as in Mignonette. In Leontice thalictroides (Blue Cohosh), species of Ophiopogon, Peliosanthes, and Stateria, the ovary ruptures immediately after flowering, and the ovules are exposed ; and in species of Cuphea the placenta ultimately bursts through the ovary and corolla, and becomes erect, bearing the exposed ovules. The ovule is attached to the placenta either directly, when it is called sessile, or by means of a prolongation called a funicuhts, umbilical cord, or podosperm (fig. 270, /). This cord sometimes becomes much elongated after fertilization. The part by which the ovule is attached to the placenta or cord is its base or hilum, the opposite extremity being its apex. The latter is frequently turned round in such a way as to approach the base. The ovule is sometimes embedded in the placenta, as in Hydnora. end ^. n Fig. 270. Fig. 272. FIG. 270. Young ovule of Celandine (Chelidonium majut) before its coverings are developed. It consists of the nucleus n. which at this stage of growth is naked. The base of the nucleus, where the nourishing vessels enter, is marked ch. This point is called the chalaza. FIG. 270a. The ovule of Polygonum, with its nucleus n, covered by the inner coat s, or the secundine, and the outer coat p, or the primine. The opening in the secundine, end, is called the endostomn, that in the primine, ex, is the exo- stome. The point of the nucleus is seen projecting at the foramen. The end by which the ovule is attached to the placenta is marked/. FIG. 271. Orthotropous or orthotropal ovule of Polygonum, showing the embryo- sac s, in the nucleus n, the different ovular coverings, the base of the nucleus or chalaza ch, and the apex of the ovule with its foramen m. FIG. 272. Vertical section of the ovule of the Austrian Pine (Finns austriaca), showing the nucleus a, consisting of delicate cellular tissue containing deep in its substance an embryo-sac b, formed before impregnation by the coales cence of a vertical series of a few cells. The micropyle m is very wide, and through it the pollen-grains come into contact with the summit of the nucleus, into the substance of which they send their tubes. The ovule appears at first as a small cellular projection from the placenta. The cells multiply until they assume a more or less enlarged ovate form, constituting what has been called the nucleus (fig. 270, n), or central cellular mass of the ovule. The nucleus may remain naked, and alone form the ovule, as in Balanophoracea;, Santalacerc, &amp;lt;tc.; but in most plants it becomes surrounded by certain coverings or integuments during its development. These appear first in the form of cellular rings at the base of the nucleus, which gradually spread over its surface. In some cases only one covering is formed, especially amongst gamopetalous Dicotyledons, as in Compositaj, Campanu- lace?e, also. in Walnut, &c. But usually besides the single covering (fig. 270a, s) another is developed subsequently (fig. 270, i&amp;gt; which gradually extends over that first formed, and ultimately covers it completely, except at the apex. There are thus two integuments to the nucleus, an outer and an inner, called respectively primine, p, and secundine, s, the terms having reference to their position as regards the nucleus and not indicating the order of development. The name tercine has been given to the outer layer of cells of the nucleus. The integuments do not completely invest the apex of the nucleus, but an opening termed the foramen or micropyle is left. This foramen in most ovules extends through both coats, the opening in the primine (iig. 270, ex) being the cxostome ; that in the secundine (fig. 270&amp;lt;r, end) being the endostome. But in many Monocotyledons the cxostome has no share in the formation of the micropyle. The micropyle indicates the organic apex of the ovule. The term micropyle is sometimes restricted to the foramen in the perfect seed. The size IV. IQ