Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/149

Rh POLLEN.] BOTANY 139 stamens are situated. In some Orchids, as Cypripedium, the pollen has its ordinary character of separate grains. The number of pollinia varies ; thus, in Orchis there are usually two, in Cattleya four, and in Lselia eight. The two pollinia in Orchis Morio contain each about 200 secondary smaller masses. These small masses, when bruised, divide into grains which are united in fours. In Asclepiadaceaj the pollinia are usually united in pairs (fig. 229), belonging to two contiguous antherine cavities, each pollen-mass having a caudicular appendage, ending in a common gland, by means of which they are attached to a process of the stigma. The pollinia are also provided with an appendicular staminal covering (fig. 230). When mature, the pollen-grain is a cellular body having an external covering, extine, and an internal, intine. Fritzsche states that he has detected, in some cases, other two coverings, which he calls intextine and exintine. They occur between the extine and intine, and are probably formed by foldings of these membranes. In some aquatics, as Zostera marina, Zannichellia pedunculata, Naias minor, &c., only one covering exists, and that is said to be the intine. The extine is a firm membrane, which defines the figure of the pollen-grain, and gives colour to it. It is either smooth, or covered with numerous projections (fig. 231), granules, points, minute hairs, or crested reticulations. The colour is generally yellow, and the surface is often covered with a viscid or oily matter. The intine is uniform in different kinds of pollen, thin and transparent, and possesses great power of extension. Pollen-grains vary from T5 -L to y^ of an inch or less in diameter. Their forms are various. The most common form of grain is ellipsoidal, more or less narrow at the extremities, which are called its poles, in contradistinction to a line equidistant from the extremities, and which is its equator. Pollen-grains are also spherical ; cylindrical and curved, as in Tradescantia virginica ; polyhedral in Dipsacaceas and Composite : nearly triangular in Proteacete and Onagraceje (fig. 232). The surface of the pollen-grain is either uniform and homogeneous, or it is marked by $&$&&amp;gt; ^.r//v Fig. 231. Fig. 232 Fig. 234. l- io. 231. Ripe round pollen of Hollyhock (Althaea), -with Its extine covered with prominent points. FIG. 232. Triangular pollen of Evening Primrose (CEnothera), with one pollen tube, t, protruded. This tube is formed by the intine, which is also seen projecting at the other angles. FIG. 233. Kipe rounded pollen of Cherry (Cerasus), discharging its fovilla through a tubular opening formed by the intine. There are other two points at which the intine is seen protruding. FIG. 234. Male flower of Pellitory (Parietana officinalis), having four stamens with incurved clastic filaments, and an abortive pistil in the centre. When the perianth expands, the filaments are thrown out with force, so as to scatter the pollen on the female flowers in the vicinity. folds dipping in towards the centre, and formed by thin nings of the membrane. In Monocotyledonous plants there is usually a single fold ; in Dicotyledons, often three. Two, four, six, and even twelve folds are also met with. There are also rounded portions of the membrane or points of perforation or pores visible in the pollen-grain. These vary in number from one to fifty. In Monocotyledons, as in Grasses, there is often only one, while in Dicotyledons they number from three upwards. When numerous, these points are either scattered irregularly, or in a regular order, frequently forming a circle round the equatorial sur face. Sometimes at the place where they exist, the outer membrane, in place of being thin and transparent, is separated in the form of a lid, thus becoming operculate, as in the Passion-flower and Gourd. Grains of pollen have sometimes both folds and pores. Within the pollen- grain a granular semifluid matter called fovilla is contained, along with some oily particles, and occasionally starch. The fovilla contains small spherical granules, sometimes the sooou- of an inch in diameter, and large ellipsoidal or elongated corpuscles, which exhibit molecular movements under the microscope. The application of moisture affects very markedly the pollen-grain, causing it to swell up by endosmose. If the moisture be long applied the distension becomes so great as to rupture the extine irregularly if it is homogeneous, or to cause projections and final rupture at the folds or pores when they exist. The intine, from its dis- tensibility, is not so liable to rupture ; it is often forced through the ruptured extine, or through the pores, in the form of small sac-like projections. The internal membrane ulti mately .gives way, and allows the granular fovilla to escape (fig. 233). If the fluid is applied only to one side of the pollen-grain, as when the pollen is applied to the pistil, the distension goes on more slowly, and the intine is prolonged outwards like a hernia, and forms an elongated tube called a pollen-tube (fig. 232). This tube, at its base, is often covered by the ruptured extine, and it contains in its interior f ovilla-granules. The number of pollen-tubes which may be produced depends on the number of pores. In some pollinia the number of tubes which are formed is enormous. Thus, it has been calculated that two pollen- masses of Orchis Morio may give out 120,000 tubes. When the pollen-grains are ripe, the anther dehisces Pollination, (figs. 1 75, 223), and the pollen is shed. In order that fertiliza tion may be effected the pollen must be conveyed to the female organ of reproduction. This process, termed pollina tion, is promoted in various ways, the whole form and structure of the flower having relation to the process. In some plants, as Kalmia and Pellitory (fig. 234), the mere elasticity of the filaments is sufficient to effect this ; in other plants pollination is effected by the wind, as in Coniferae, and in such cases enormous quantities of pollen are produced. These plants are anemoplrilous. But the com mon agents for pollination are insects. To allure and attract them to visit the flower the odoriferous secretions and gay colours are developed, and the position and complicated structure of the parts of the flower are adapted to the perfect performance of the process. It is rare in hermaphrodite flowers for self-fertilization to occur, and the various forms of dichogamy, dimorphism, and trimorphism are fitted to prevent this. Under the term disk is included every structure inter- Disk, vening between the stamens and the pistil. It was to such structures that the name of nectary was applied by old authors. It presents great varieties of form, such as a ring, scales, glands, hairs, petaloid appendages, &c., and in the progress of growth it often contains sac charine matter, thus becoming truly nectariferous. The disk is frequently formed by degeneration or transformation of the staminal row. It may consist of processes rising from the torus, alternating with the stamens, and thus representing an abortive whorl ; or its parts may be oppo-