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Encyclopedia Britannica - Main :: RON-SAC |
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ROSACEAE , in botany, ,a large cosmopolitan family of seed-bearing plants belonging to the subclass Polypetalae of Dicotyledons and containing about 90 genera with 2000 species. The plants vary widely in manner of growth. Many are herbaceous, growing erect, as Geum
The leaves, which are arranged alternately, are simple, as in apple, cherry, &c., but more often compound, with leaflets palmately arranged, as in strawberry and species of Potentilla, or pinnately arranged, as in the brambles, roses, mountain ash, &c. A difference in this respect often occurs in one and the same genus, as in Pyrus, where apple (P. Malus), and pear (P. communis) have simple leaves, whereas mountain ash or rowan (P. aucuparia) has pinnately compound leaves. Inwarm climates the leaves are often leathery and evergreen. The leaves are stipulate, the stipules being sometimes small and shortlived, as in Pyrus and Prunus (cherry, plum, &c.), or more important structures adnate to the base of the leaf-stalk, as in roses, brambles, &c. The flowers
agrimony
flowers
features of the suborders, six of which may be different recognized. I. Spiraeoideae is characterized by a flat or slightly Suborder concave receptacle on which the carpels, frequently five in number, form a central whorl; each ovary contains several ovules, and the fruit is a follicle. There are five sepals, five petals and the stamens vary from ten to indefinite. The plants are generally shrubs with simple or compound leaves and racemes or panicles of numerous small white, rose or purple flowers. This suborder, which is nearly allied to the order Saxifragaceae
Suborder II. Pomoideae is characterized by a deep cup-shaped receptacle with the inner wall
Suborder III. Rosoideae is characterized by the receptacle being convex and swollen (fig. 1, I), as in strawberry, or' cup-shaped, as in rose (fig. 4), and bearing numerous carpels, each of which contains one or two ovules, while the fruit is one-seeded and indehiscent. The 39 genera are grouped in tribes according to the form of the receptacle and of the fruit. The Potentilleae bear the carpels on a large, rounded or convex outgrowth of the receptacle. In the large genus Rubus (fig. 3) the ripe ovaries form drupels upon the dry receptacle; the genus is almost cosmopolitan, but the majority of species occur in the forest region of the north temperate zone and in the mountains of tropical America. R. fruticosus is blackberry, R. Idaeus, raspberry, and R. Chamaemorus, cloudberry. In the flower of Potentilla, Fragaria (straw-berry) and a few allied genera an epicalyx is formed by stipular structures arising at the base of the sepals. The fruits consist of numerous dry achenes borne in Fragaria on the much-enlarged PJlanzenjamilien, from Strasburger's Le/aback der Bolanik, by permission of Gustav Fischer. 1 in Naliirl. After Focke succulent torus, which in the other genera is dry. In Geum
After Wossidlo, from Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer. fruit, forming a feathery appendage (Dryas) or a barbed awn (avens), either of which is of service in distributing the fruit. The Potentilleae are chiefly north temperate, arctic and alpine plants. After Wossidlo, from Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer. The Roseae comprise the large genus Rosa, characterized by a more or less urn-shaped torus (fig. 4) enclosing the numerous carpels which form dry one-seeded fruits enveloped in the bright -coloured fleshy torus. The numerous stamens surround the mouth of the torus. The plants are shrubs bearing prickles on the stems and leaves; many species have a scrambling habit resembling the brambles. The species of Rosa, like those of Rubus, are extremely variable, and a great number of subspecies, varieties and forms have been described. The Sanguisorbeae are a reduced form of Rosoideae. The dry one-seeded fruit is enclosed in the urn-shapedtorus, which, however, is dry and inconspicuous, and the number of carpels is much reduced, sometimes to one (figs. 2, 5, 6). Petals are often wanting, as in Alchemilla (lady's r..r, mantle) and Paterium, and the flowers are often unisexual and frequently wind-pollinated, as in salad burnet (Poterium Sanguisorba), where the small flowers are crowded in heads, the upper pistil-late, with protruding feathery stigmas, and the lower staminate (or bisexual), with exserted stamens. Agrimonia (agrimony
Suborder IV. Neuradoideae contains only two genera of desert-inhabiting herbs with yellow flowers; and the five to ten carpels are united together and with the base of the cup-shaped torus, which enlarges to form a dry covering round the one-seeded fruits. Suborder V. Prunoideae (fig. 7) is characterized by a free solitary carpel with a terminal style and two pendulous ovules, and the fruit a one-seeded drupe. The torus forms a cup from the edge of which spring the five sepals, five alter-nating petals and the ten to indefinite stamens. The plants are deciduous or evergreen trees or shrubs with simple leaves, often with small caducous stipules, and racemes or umbels of generally showy, white or pink
chief
After Wossidlo from Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer. through longitudinally; 3, fruit in longitudinal section. laurel (P. Laurocerasus). The tribe is distributed through the north temperate zone, passing into the tropics. Suborder VI. Chrysobalanoideae resembles the last in having a single free carpel and the fruit a drupe, but differs in having the style basal, not terminal, and the ovules ascending, not pendulous; the flowers are also frequently zygomorphic. The 12 genera are tropical evergreen trees or shrubs, the great majority being South American. The zygomorphic flowers indicate an affinity with the closely allied order Leguminosae. B After Duchartre, from Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer. 6 gram of Sanguisorba. b, bract; a', O', bracte- oles; d, disk. End of Article: ROSACEAE If you wish, you can link directly to this article.
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