SPORE, STEM., etc. We shall therefore here only discuss one subject, which has not been separately considered—namely, the organs and functions of reproduction in plants. Although, as we learn from Herodotus, the Babylonians knew that there were male and female date-trees, and that the female required the concurrence of the male to become fertile, and Theophrastus in his work On the History of Plants, and other ancient authors, frequently mention the sexes of plants, Cmsalpinits, who died at Rome in 1603, seems to have been the first writer who directed his attention to the reproductive organs of plants; and he speaks vaguely of an emanation from the male causing fertility in the female; and Grew, in 1676, seems to have been the first who distinctly recognized the functions of the stamens and pistils. Ray, in his Historia Plantarum, 1694, adopted and enforced Grew's view; and Geoffroy, in 1711, read a memoir before the royal academy support ing the same view. Linmens, in his Systema Nature (1748), made these organs the foundation of his system of classification into sexual and non-sexual plants, the former being phanerogamous, or flowering, and the latter cryptogamous, or flowerless; in the latter division of plants, he could not detect stamens or pistils; and it was not till 1782, when Hedwig's work on mosses was published, that anything was known with certainty regarding the sexual organs of any of the cryptogamia. From this brief notice of the early history of this subject, we proceed to the consideration of reproduction in the phanerogamous plants. A complete flower consists, as is well known, of four whorls (verticals), placed alternately within one another, the two internal being the stamens (q.v.) and pistils (q.v.),which are the essential organs of reproduction; while the two external are the calyx and corolla, which constitute the floral envelopes or protective coverings. Both the stamens and the pistils originate, like the floral lopes (see FLOWER), from the thalamus, or upper part of the axis or peduncle, in the form of minute cellular processes; and in their development they resemble leaves, although in their appearance, they are less like leaves than are the floral lopes. These parts arc well seen in the accompanying diagram (fig. 1) of the flower of the vine, after it has cast its petals. There are here five stamens (the filament of one being concealed by the pistil), with introrse * two-lobed anthers. As separate articles are devoted to STAMENS and PISTILS, it is unnecessary to enter into any details regarding their anatomical structure. A few additional remarks ou the pollen are, however, called for. This (tile male fertilizing agent) consists of cells ed in the anther case, and is discharged by various kinds of longitudinal,transverse, valvular, or porous dehiscence. When examined by the naked eye, it usually appears as a yellow powder; but when magnified, it is found to consist of eellules of different singular forms, varying in size from spa to of an inch in diameter. Oval, spherical, and triangular forms of pollen are shown in figs. 2 to 6; and they may be square cylindrical hour-glass shaped, etc. fhese pollen•grains are developed in the large cells in the early stage of the anther. The contents of each cell divide first into two, and afterward into four parts, each of which becomes covered with cellulose, so as to constitute independent cells or grains. These grains either burst through the parent cell, and become liberated, or they remain united in fours or some multiple of four, as in many species of acacia; or, in large masses, such as those seen in orchids and in asclepias, when they constitute pollinia. Each pollen-grain has usually two coverings: the outer one, called ertine, being a firm membrane, often marked with bands or rough points; and the inner one called intine, which is thin, and capable of extension. In the interior of the pollen-grains, a minute granular matter exists, called granules, which are mixed with starch and oil, varying fromto , of an inch in diameter. On moistening pollen-grains in water, they swell tittte intine bursts at one or more points, and expels the fovilla. In the act of impregnation the pollen is scattered on the pistil, and is moistened on one side by the fluid of the stigma (a part of the pistil com posed of loose cells, which secrete a viscid fluid, and are uncovered by epidermis). It is then observed that the intine, instead of bursting, protrudes in the form of a tube called the The number of these tubes varies greatly in different plants.
According to Amici (as quoted by Balfour, to whose useful we are indebted for most of our facts and illustrations), the two pollinia of orehis morio contain each about 200 secondary small masses, composed of grains united in fours, and each of these small masses presents 300 openings capable of emitting tubes. In order that an embryo plant may be formed, the mature pollen must be discharged from the anther cells of the sta men, and brought into contact with the stigma, through which, and then through the conducting tissue of the style, it must pass until it reaches the foramen, or micropyle, of the ovule. The means by which this contact is accomplished are various, such as elas ticity and irritability of the stamens, the action of currents of air, and the intervention of insects passing from the male to the female plant. In the case of the orchids, fertili zation is solely effected by the agency of insects. The fertilizing power of pollen -is retained for a different length of time in different plants: thus, while in most species of datum, and in lychnis diolea, it loses its power in two days, in the wall-flower it remains efficacious for 14 days; while in the date, cannabis, tea, and camellia, it will keep fresh for a year; indeed, 31i8heaux mentions that the pollen of the date has been successfully used after 18 years! The quantity of pollen that is produced is much greater than is actually required for the irnpreguation of the ovules. Thus, in the firs' and pines, the quantity is enormous, probably becauso of the obstacles here presented to fertilization. The sulphur showers occurring in some districts are composed of the yellow pollen car ried by the winds from pine forests; and the showers of colored rain which are occasion ally noticed are due to a similar cause. The number of pollen-grains in certain flowers has been calculated. In a plant of areas granclifforus, Morren observed that there were 40 flowers, each containing 500 stamens, and that each anther contained 500 pollen grains; hence the entire number of pollen-grains in each flower was 250,000, and in the whole plant its 10,000,000. Similarly, in an entire rhododendron plant, the pollen-grains amount to 72,620,000. The quantity required for fertilization is very small—one, two, or at most three grains being sufficient to impregnate one ovule. In most cases, the pol len of a single anther is sufficient for complete impregnation; the additional anthers being, as it were, added for the purpose of insuring the result. During the evolution of the stamens, and the maturation of the pollen, the. pistil undergoes certain changes, of which the most important is that the stigma becomes enlarged, lax, and covered with a viscid secretion, which, besides detaining the pollen-grains, causes them to protrude their tubes, as already described; moreover, in some flowers the style, which is sometimes covered with hairs, elongates during the discharge of the pollen, brushes the latter on to the pistil, and thus acts directly in fecundation. Oue of the central cells of the ovule now becomes much enlarged and developed, so as to form the embryo sac. At the end of this sac, next to the micropyle, several free nucleated cells are formed, to which the name of embryo vesicles, or germinal vesicles, has been given. In this way the ovule is prepared for the action of the pollen, and for the production of the embryo plant. The tubes developed by the pollen-grains, when acted on by the secretion of the stigma, pierce the stigmatic tissue, and carry the fovilla through the canal of the style to the ovule, as shown in fig. 7. In some plants the emission of tubes begins in half a minute after the pollen has been caught by the stigmatic secretiou; in other cases it does not begin for 24 hours or more; and it is said that in the larch, the tubes do not emerge for 35 days. The length to which the tubes extend is often very great, but the diameter is extremely small. In colchicum autammule, in which the style is 13 in. long, the length of the tube is 9,000 times the diameter of the grain from which it proceeds. The time taken by the tube to traverse the length of the style varies, but does not always correspond with the latter. In some short-styled plants, the time is very long, while in the long-styled colchicum uutumwtle, the pollen-tube reaches the ovule iu about 12 hours. In some coniferous plants a year is required for the process.