Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/148

Rh 138 BOTANY [REPRODUCTIVE ORGANS. Plate XI. hinged. In the Barberry (fig. 225) each lobe opens by a valve on the oxiter side of the suture, separately rolling up from base to apex ; in some of the Laiirel tribe there are two such valves for each lobe, or four in all. This may be called a combination of transverse and hinged dehiscence. In some Guttifera3, as Hebradendron cambogioides (the Ceylon Gamboge plant), the anther opens by a lid separat ing from the apex ; this is called circumscissile dehiscence. The anthers dehisce at different periods during the pro cess of flowering ; sometimes in the bud, but more com monly when the pistil is fully developed, and the flower is expanded. They either dehisce simultaneously or in succession. In the latter case individual stamens may move in succession towards the pistil and discharge their contents, as in Parnassia palustris, or the outer or the inner stamens may first dehisce, following thus a centri petal or centrifugal order. The anthers are called introrse or anticce, when they dehisce by the surface next to the centre of the flower ; they are extrorse, or posticce, when they dehisce by the outer surface ; when they dehisce by the sides, as in Iris and some grasses, they are laterally dehiscent. Sometimes, from, their versatile nature, anthers originally introrse become extrorse, as in the Passion flower and Oxalis. The usual colour of anthers is yellow, but they present a great variety in this respect. They are red in the Peach, dark purple in the Poppy and Tulip, orange in Esch- scholtzia, &c. The colour and appearance of the anthers often change after they have discharged their functions. Stamens occasionally become sterile by the degeneration or non-development of the anthers. Such stamens receive the name of staminodia, or rudimentary stamens. In Scrophularia the fifth stamen appears in the form of a scale ; and in many Pentstemons it is reduced to a filament with hairs or a shrivelled membrane at the apex. In other cases, as in double flowers, the stamens are con verted into petals ; this is also probably the case with such plants as Mesembryanthemum, where there is a multiplica tion of petals in several rows. Sometimes, as in Canna, one of the anther-lobes becomes abortive, and a petaloid appendage is produced. Stamens vary in length as regards the corolla. Some are enclosed within the tube of the flower, as in Cinchona, and are called included; others are exserted, or extend beyond the flower, as in Littorella, Plantago, and Exostemma. Sometimes the stamens in the early state of the flower project beyond the petals, and in the progress of growtli become included, as in Geranium striatum. Stamens also vary in their relative lengths as respects each other. When there is more than one row or whorl in a flower, those on the outside are sometimes longest, as in many Rosacese ; at other times those in the in terior are longest, as in Luhea. When the stamens are in two rows, those opposite the petals are usually shorter than those which alternate with the petals. It sometimes happens that a single stamen is longer than all the rest. A definite relation, as regards number, sometimes exists between the long and the short stamens. Thus, in some flowers the stamens are didynamous, having only four out of five stamens developed, and the two corresponding to the upper part of the flower longer than the two lateral ones. This occurs in Labiatae and Scrophulariaceje (fig. 226). Again,- in other cases, there are six stamens, whereof four long ones are arranged in pairs opposite to each other, and alternate with two isolated short ones (fig. _227), giving rise to tetradynamous flowers, as in Crucifene. Stamens, as regards their direction, may be erect, turned inwards, outwards, or to one side. In the last-mentioned case they are called dedinate, as in Amaryllis, Horse-chestnut, and Fraxinella. The pollen or powdery matter contained in the anther consists of small cells developed in the interior of other Pollen, cells. These are produced in the large thick-walled mother- Fig. 22t&amp;gt;. Fi- 2 7 J. 1_. ftit I. FIG. 22G. Corolla of Digitalis purpurea, cut in order to show the didynamous stamens (two long and two short) which are attached to it. t, tube ; /, filaments which are united to the corolla at j, and run along its inner surface, having formed a marked adhesion; ag, anthers of the long stamens; aq, anthers of the short stamens. FIG. 227. Tetradynamous stamens (four long and two short) of Cfieiranthus Cheiri. p, top of the peduncle ; o, cicatrices left by the sepals of calyx which have been removed; eg, .two pairs of long stamens; ep, the short stamens; t, torus or thalamus to which the stamens are attached cells or pollen utricles, formed in the interior of the pollen- sacs of the young anther. These mother-cells are either separated from one another and float in the granular fluid which fills up the cavity of the pollen-sac, or are not so isolated. A division takes place, by which four cells are formed in each, the exact mode of division differing in Dicotyledons and Monocotyledons. These cells are the pollen-cells or grains. They increase in size and acquire a cell-wall, which becomes differentiated into an outer, cuticular layer, or extine, and an inner layer, or intine. Then the walls of the mother-cells are absorbed, and the pollen-grains float freely in the fluid of the pollen- sacs, which gradually disappears, and the mature grains form a powdery mass within the anther. They then either remain united in fours, or multiples of four, as in some Acacias, Periploca graca, and Inga anomala, or separate into individual grains, which by degrees become mature pollen. Occasionally the membrane of the pollen-cell is not com pletely absorbed, and traces of it are detected in a viscous matter surrounding the pollen-grains, as in Onagracea}. In Orchidaceous plants the pollen-grains are united into masses, or pollinia (fig. 228), by means of viscid matter. Fig. 228. Fig. 230. FIG. 228. Pollinia, or pollen-masses, separated from the point above the stigma, with their retinacula or viscid matter attaching them at the base. The pollen masses^) are supported on stalks or caudiclesc. These masses are easily detached by the agency of insects. FIG. 229. Pistil of Asclcpias, a, with pollen-masses, p adhering to the stigma s. 6, pollen-masses, removed from the stigma, united by a gland-like body. FIG. 230. Stamen of Asclepias, showing filament /, anther a, and appendages P- In Orchids each of the pollen-masses has a prolongation or stalk, called a caudicle, which adheres to a prolongation at the base of the anther, called rostellum, by means of a viscous gland, called retinaculum. The gland is either naked or covered. The term clinandrium is sometimes applied to the part of the column in Orchids where the