PLANT 587 (adnate), or it may open toward the pistil (in- trorse), or away from it (extrorse), and again it may, as in the lily, be hung by the centre and move in any direction (versatile). The various shapes assumed by the two lobes are used in descriptive botany, as are the methods by which the pollen is liberated; instead of the usual longitudinal slit, each cell may have a pore at the top, or each may have, as in the barberry, one, or as in the sassafras and others of the laurel family, two valves, or trap doors, hinged at the top, which open to liberate the pollen. In respect to numbers, they range all the way from a solitary stamen to so many that the number is regarded as indefinite; when of the same number as the petals, they are usu- ally alternate with them ; when twice as many, then one is opposite the petal and the other al- ternate. The stamens sometimes appear in two or more definite series ; often a certain number have no anthers and are abortive. As with the sepals and petals, the stamens are often united ; when their filaments cohere to form a tube (e. g., the mallow family), they are monadel- phous ; if in two groups (e. g., pea), they are diadelphous ; and they may unite in threes or larger clusters. Where the union takes place by the anthers, leaving the filaments free, the stamens are syngenesious. When the anther has reached its full development it opens in one way or another for the exit of the pollen, which is usually a loose powder, most frequent- ly yellow, but sometimes brown, purplish, blu- ish, &o. ; usually globular or oval, it is some- times polyhedral, in the pine of three, and in the cat-tail of four united grains. In many or- chids it is bound together in pollinia by elastic threads (see OECHIDS), and in the milkweeds (Asclepias) it appears as solid waxy masses. In size the pollen grains vary from -^ millime- tre to Y-Q-J or even less. Some kinds of pollen have a surface beautifully marked with spines, or by the unequal thickening of the membrane. In structure the grain consists of an outer (extine) and an inner (intine) membrane, and is within filled with a thick granular liquid, the fomlla, which is its essential part. When pollen is moistened under the microscope, the grains swell and burst, either by slits or by pores, which are thinner parts of the outer membrane, and through these the inner mem- brane protrudes in what is called the pollen tube, which in turn soon bursts and sets free the fovilla. The number of openings in the outer membrane of the pollen grain varies from one to several, though but one is used. The pistil (or gyncecium) occupies the centre of the flower, and to the nourishment, protection, and development of this all other parts are sub- servient. Its parts, ovary, style, and stigma, have been already pointed out. In the flower chosen for illustration there are five separate and distinct simple pistils, and these are ar- ranged in a circle in the centre of the flower ; often there is but one, which then occupies the centre, or if there are two, they stand face to face in the centre; its occasional eleva- tion upon a prolonged receptacle has been re- ferred to. In the pistil already illustrated the parts are not so distinct as in some others. The ovary and stigma are the essential portions, and the style is frequently not manifest, the stigma being sessile directly upon the ovary (e. g., poppy), but in many cases it is much elon- gated, and as in colchicum several inches long, The pistil already referred to is what is called a simple pistil or carpel, having but one ova- ry, one style, and one stigma. As with other parts of the flower, the pistils sometimes co- here to form a compound pistil; this union may involve the ovaries only, or even only the lower parts of them, leaving the styles distinct; it may include the styles, the stigmas remain- ing separate; or all may be united through- out. The stigma, if terminal, may be a simple knob; it is sometimes lobed, and in some cases, as in Sarracenia, forms an umbrella-like body, with stigmatic points at the margin. (See PITCHEE PLANTS.) In some flowers it is not con- spicuous, but simply a space denuded of epi- dermis ; and it generally has a viscid secretion to hold the pollen which may fall upon it. The ovary of a compound pistil, when trans- versely divided, usually shows as many cells as there are simple pistils entering into its com- position ; if the five simple pistils of our typi- cal flower were to be consolidated into one, it would bring the placentae or ovule-bearing portions in the centre, and this position (axile) of the placentaa is very common in compound pistils; but in some flowers (e. g., poppy) a compound pistil may be one-celled, with its placenta in lines upon the wall of the ovary, a peculiarity of structure to be hereafter ex- plained. In the compound pistil formed by the union of simple ones, or carpels, the divi- sion walls of the cells, extending from the circumference to the centre, are structurally double, as may often be seen in the fruit. The Ovule and Fertilization. A section of the simple pistil or carpel of the typical flower shows the ovules at one side; they are at- tached to two lines of loose tissue, the placen- ta, and in this case are numerous ; often the ovule is solitary ; sometimes there is a definite number, and again so many as to be indefinite. It has been already intimated that the ovule is the undeveloped seed, but more strictly speak- ing it is the organ within which the seed is to be formed; it at first appears as a minute papilla or pulpy swelling upon the placenta, the nucleus ; soon a coating grows outside of and envelops this, which is soon followed by a second and similar coat, thus enclosing the nucleus within two sacs, having at the top cor- responding openings, the foramen or micro- pyle ; the ovule is supported upon a longer or shorter stalk, the funiculus. The ovule may be straight, bent over on itself, or bent down upon its long stalk so that it is completely in- verted. When it is fully developed a cavity is formed in or near its centre, the embryonic
