Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/145

STAMENS.] BOTANY 135 order that perfect seed may bo produced. A flower may have a calyx and corolla, but it will be imperfect if the essen tial organs are not present. The name of Jiermaphioditc or bisexual is given to flowers in which both these organs are found ; that of unisexual or diclinous to those in which only one of these organs appears, those bearing stamens only, being staniiniferous or male ; those having the pistil only, pistilliferous or female. But even in plants with hermaphrodite flowers it is rare that self-fertilization takes place, and this is provided against by the structure of the parts or by the period of ripening of the organs. For instance, in Primula and Linuiu some flowers have long stamens and a pistil with a short style, the others having short stamens and a pistil with a long style. The former occur in what are called thrum-eyed Primroses, the latter in those called pin-eyed. Such plants are called dimorphic. Other plants are trimorpkic, as species of Lythrum, and proper fertilization is only effected by com bination of parts of equal length. In some plants the stamens are perfected before the pistil ; they are called proterandrous, as in Ranunculus repens, Silene maritima, Zea Mays. In other plants the pistil is perfected before the stamens, as iu Potentilla argentea, Plantago major, Coix Lachryma, and they are protcroyi/uous plants. Plants in which proterandry or proterogyny occurs are called dicho- jamous. When in the same plant there are unisexual flowers, both male and female, the plant is said to be monoecious or monoicous, as in the Hazel and Castor-oil plant. This is indicated by the symbol 6-9. When the male and female flowers of a species are found on separate plants, the term dioecious or dioicous is applied, as in Mercurial is and Hemp, and the symbol J : 5 is used; and when a species has male, female, and hermaphrodite flowers on the same or different plants, as in Parietaria, it is polygamous, for which J 9 is the symbol. 1. Male Organs in Phanerogams. The stamens (stamina] arise from the thalamua or torus within the petals, with which they alternate, forming one or more verticils or whorls, which collectively constitute the androecium, or the male organs of the plant. Their normal position is below the inner whorl or the pistil, and when they are so placed (fig. 215, e) upon the thalamus they arc hypogynous. Sometimes they become adherent to the petals, or are epipctalous, and the insertion of both is looked upon as similar, so that they are still hypogynous provided they are independent of the calyx and the pistil. Frequently the margins of the thalamus bearing the floral envelopes and stamens elongates, and the gyucecium remains in the centre of the concave receptacle ; thus the stamens aa it were rise from the calyx, and they surround the ovary, and are perigynous ; but when the ovary becomes completely inferior by the growing upwards and inwards of the receptacle, the parts of the flower rising from its summit, the stamens are epigynous (fig. 2 1C). Numerous inter mediate forms occur, especially amongst Saxifragacere, where the parts are half superior or half inferior. In the Orchis tribe, where the stamens become adherent to the pistil so as to form a column, the flowers are said to be gynandrous. The same is the case in Aristolochia (fig. 217). These arrangements of parts are of great importance in classifica tion. The stamens vary in number, from one to many hun dreds. Like the other parts of the flower they are modified leaves, resembling leaves in their structure, development, and arrangement. They consist of cellular and vascular tissues. They appear at first in the form of cellular projections, and are arranged in a more or less spiral form. In their general aspect they have a greater resemblance to petals than to the leaves, and there is often seen a gradual transi tion from petals to stamens, especially in spiral flowers, as Nymphcea alba. When flowers become double by cultiva tion, the stamens are converted into petals, as in the Pax>ny, Camellia, Hose, itc. When there s or.lv one whorl the Fte. 215. Fig. 217. FIG. 215. Section of a flower of Geranium Rolertianum. c, c, calyx ; p, petals ; , stamens. The pistil is composed of ovary o, and style and stigmata s. t is the torus or thalamus. The petals and stamens are hypogynous, and the latter are rnonadelphous. Fin. 216. Section of the flower of Aralia spinosa. Letters as in last figure. Tho petals and stamens are epigynous, attached to the torus rf, which covers the summit of the ovary. The ovary is adherent to the torus, and has been laid open to show its loculaments and pendulous ovules. FIG. 217. Essential organs of Birthwort (Aristolochia). Above the ovary 6, nre the stamens united in a column with the style at a, so as to be gynandrous. On the summit is the stigma above the stamens. stamens are usually equal in number to the sepals or petals, and are arranged opposite to the former, and alternate with the latter. The flower is then isostemonous. When the stamens are not equal in number to the sepals or petals, the flower is anisostemonous. When there is more than one whorl of stamens, then the parts of each suc cessive whorl alternate with those of the whorl preceding it. The staminal row is more liable to multiplication of parts than the outer whorls. A flower with a single row of stamens is aplostemonous. If the stamens are double the sepals or petals as regards number, the flower is diplostemonous ; if more than double, polystemonous. The additional rows of stamens may be developed in the usual centripetal (acropetal) order, as in Rhamnacese ; or they may be interposed between the pre-existing ones or be placed outside them, i.e., develop centrifugally (basipetally), as in Geranium and Oxalis. When the stamens are neither equal to, nor a multiple of, the floral envelopes, but are less numerous, the flower is miostemonous. When the stamens are fewer than twenty they are said to be definite, and the flower is oligandrous ; when above twenty they are indefinite or polyandrous, and are represented by the symbol co. The number of stamens is indicated by the Greek numerals prefixed to the term androus ; thus a flower with 1 stamen is Monandvous (Hippuris, Centvanthus) ; 2 stamens is Diandrous (Veronica, Calceolaria, Circtta) ; 3 stamens is Triandrous (Grasses, Iris, Valeriana) ; 4 stamens is Tetraiidrons (Alchemilla, Galium, Plantago) ; 5 stamens is Pcntandrous (Primula, Umbelliferffl, Campanula) ; 6 stamens is Hexandrous (Tulip, Lilium, Juncus) ; 7 stamens is Heptandrous (Trientalis, Horse-chestnut) : 8 stamens is Octandrous (Heath, Fuchsia, Epilobium) ; 9 stamens is Enneandrous (Butonms, some Lauraceaj) ; 10 stamens is Decandrous (Saxifraga, Dianthus, Oxalis) ; 12 stamens is Dodecandvous (Asarum, Agrimonia, Ueseda) ; 20 stamens is Icosandrous (Strawberry, Potentilla, Crataegus) ; Numerous and indefinite stamens is Polyandrous < (Poppy). The simplest form of stamens is seen in Cycadaceous plants, where cataphyllary leaves (the scales of the cone) Plate XV. are the staminal leaves, bearing the pollen-sacs scattered over their under .surface. The stamen usually consists of two

parts, a contracted portion, often thread-like, termed the