Online Encyclopedia

ROSACEAE

Online Encyclopedia
Originally appearing in Volume V23, Page 724 of the 1911 Encyclopedia Britannica.
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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, or with slender creeping stem, as in species of Potentilla, sometimes sending out long runners, as in strawberry; others are shrubby, as raspberry, often associated with a scrambling habit, as in the brambles and roses, while apple, cherry, pear, plum and other British fruit trees represent the arborescent habit. Vegetative propagation takes place by means of runners, which root at the apex and form a new plant, as in strawberry; by suckers springing from the base of the shoot and rising to form new leafy shoots after running for some distance beneath the soil, as in raspberry; or by shoots produced from the roots, as in cherry or plum. The scrambling of the brambles and roses is effected by means of prickles on the branches and leaf-stalks. 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, which are regular, generally bisexual, and often showy, are sometimes borne singly, as in some species of rose, or of the cloudberry (Rubus chamaemorus), or few or more together in a corymbose manner, as in some roses, hawthorn and others. The inflorescence in agrimony is a raceme, in Poterium a dense-flowered spike, in Spiraea a number of cymes arranged in a corymb. The parts of the flowers are arranged on a 5-merous plan, with generally considerable increase in the number of stamens and carpels. The shape of the thalamus or floral receptacle, and the relative position and number of the stamens and carpels and the character of the fruit, vary widely and form distinguishing 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, contains 17 genera, chiefly north temperate in distribution. The largest is Spiraea, numerous species of which are cultivated in gardens; S. salicifolia occurs in Britain apparently wild in plantations, but is not indigenous. The native British meadow-sweet (S. Ulmaria) and dropwort (S. Filipendula) have been placed in a separate genus, Ulmaria, and included in the Rosoideae on account of their one-seeded fruit. Quillaja sapondria is the Chilean soap tree; the bark contains saponin. Suborder II. Pomoideae is characterized by a deep cup-shaped receptacle with the inner wall of which the five or fewer carpels are united (fig. 1, 3) ; the carpels are also united with each other, and each contains generally two ovules. The fruit is made up of the large fleshy receptacle surrounding the ripe ovaries, the endocarp of which is leathery or stony and contains one seed. The plants are shrubs or trees with simple or pinnately compound leaves and white or rose-coloured often showy flowers, with five sepals and petals and indefinite stamens. The 14 genera are distributed through the north temperate zone, extending southwards in the New World to the Andes of Peru and Chile. The largest genus, Pyrus, with about 50 species, includes apple (P. Malus), pear (P. communis) (fig. 2), wild service (P. torminalis), rowan or mountain-ash (P. aucuparia), and white beam (P. Aria). Mespilus (medlar) and Cotoneaster are also included. (See separate articles for most of the-above.) 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 (avens) and Dryas (an arctic and alpine genus) the style is persistent in the 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) has a long spike of small honeyless flowers with yellow petals; in the fruit the torus becomes hard and crowned by hooked bristles which ensure the distribution of the enclosed achenes. 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 flowers. There are five genera, the chief of which is Prunus, to which belong the plum (Prunus communis), with several well-marked subspecies—P. spinosa (sloe or blackthorn), P. insititia (bullace), P. domestica (wild plum), the almond (P. Amygdalus), with the nearly allied peach (P. persica), cherry (P. Cerasus), birdcherry (P. Padus) and cherry 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
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