The flowers are regular, hypogynous, nearly always unisexual, small and insignificant, with single, green perianth of 4–5 leaves. Stamens 4–5, placed opposite the leaves of the perianth. Ovary formed of 1 or 2 carpels, most frequently unilocular, with one ovule (Fig. [340]). The fruit is a nut, more rarely a drupe, with one seed, generally endospermous. The Nettles are the sole order in the family which has only one carpel (1 stigma); this turns the posterior side to the front (Fig. [340]). The others have two carpels (2 stigmas) but the anterior only is fertile (Fig. [346]) except in a few Ulmaceæ and Moraceæ.
The majority are trees or shrubs with petiolated leaves, stipulate; rough hairs are very frequently developed upon the leaves. The flowers are very often crowded together in the inflorescence, which is rarely catkin-like. Peculiar aggregations of fruits are found in some orders. Latex and tough bast, which is used technically, are also frequently found. Cystoliths are found in the epidermis of many species of Ficus, Urtica, and others. Wind- or self-pollination is most common, as in the Quercifloræ and Juglandifloræ. In the Urticaceæ, Morus and some others, the stamens lie incurved in the bud, and when ripe straighten themselves suddenly and elastically, and thus small clouds of pollen-grains are ejected with considerable violence on to the stigmas, which are often provided with brush-like hairs (Fig. [341]). The formation of honey does not take place.
Order 1. Ulmaceæ (Elms).—Trees or shrubs without latex. Leaves simple, arranged in two rows (divergence 1/2), oblique (the inner side, nearer the axis, being the larger), strongly penninerved, dentate, hispid; stipules deciduous. In opposition to the other Nettle-like plants the flowers are often ☿ with a united cup- or saucer-like, generally 4–(5)–6-divided perianth, and a corresponding or larger number of opposite erect stamens. The gynœceum has two carpels (2 stigmas), generally one loculus with one pendulous, anatropous or amphitropous ovule,[35] seldom two loculi and 2 ovules. Fruit one-seeded (nut or drupe). Embryo without endosperm.
A. Ulmeæ. The fruit is a winged nut (Fig. [339]), the embryo straight, without endosperm. Anthers extrorse.—Ulmus (Elm). The flowers are situated in inflorescences which develop from the lower buds of the shoot of the preceding year. The lowermost bud-scales are empty, the uppermost support either solitary flowers, or small, dichasial or unipared scorpioid inflorescences. The terminal bud on the vegetative shoot quickly falls off, and the upper lateral bud continues the growth sympodially. Flowering takes place before the leaf-buds open. The flowers are wind-pollinated and have no honey. Fossil species have been found in the Oligocene.
20 species; North Temp. (2 species in this country). Important as timber. The Cork-elm (U. suberosa) has a rather thick cork, which, however, is of no technical use. The bast is used as Lime-bast.
B. Celtideæ. The fruit is a drupe, the embryo curved, with folded or rolled up cotyledons, with or without endosperm. The anthers are introrse. The flowers are borne on a shoot of the same year. Planera (N. America); Zelkova.—About 114 species; especially N. Temp., Trop.
Fig. 339.—A Ulmus campestris, flower with exceptionally aborted gynœceum; B, U. effusa, flower with 8 stamens; C, U. campestris, fruit opened in front to show the seed pendulous from the apex of the loculus; one loculus is aborted.
Order 2. Urticaceæ (Nettles).—The majority of species are herbs with simple, stipulate leaves; they have no latex; stinging hairs abundant. The flowers (Fig. [340]) are unisexual, generally 2-merous and arranged in clusters, which are united into catkin-like inflorescences. The perianth is composed very often of 4 (2 + 2) free, or in the ♀-flowers generally united, green leaves; the 4 (2 + 2) stamens are opposite the perianth-leaves, the filaments are bent inwards in the bud and throw themselves elastically towards the outside. The gynœceum has one style and one stigma (capitate or brush-like, Fig. [341]); the ovary is unilocular, with one orthotropous, erect ovule (all other orders of this family have inverted or curved ovules). Fruit, a nut or drupe. Endosperm present (in Urtica very little), oily. Embryo straight. The STINGING HAIRS are club-shaped, very turgid, and provided with a siliceous, brittle apex, which breaks off in an oblique direction and allows the poisonous cell-sap to be forced out. In many tropical Nettles this is so strong that it may produce partial paralysis. There is no rudiment of an ovary in the ♂-flowers (Fig. [340] A). The PERIANTH in the ♀-flower differs from that of the ♂ in having the two internal leaves generally much larger and enveloping the fruit (Fig. [340] B); it often happens that all the perianth-leaves are united to form a gamophyllous envelope. ☿-flowers may occur among the others.—The inflorescences among our native species are dichasia, which become transformed into unilateral scorpioid cymes by the development of the bud of the 2nd bracteole. In Parietaria they are more pressed together, and the floral-leaves at the same time are also raised on their axillary shoots to just beneath the flower. As a rule, not only in this order but also in those related to it, a small vegetative branch is situated in the axil of the foliage-leaf, and this bears an inflorescence on each side at its base.
Urtica (Nettle) has opposite leaves with distinct stipules and stinging hairs. The perianth-leaves of the ♀-flower are free (Fig. [340]).—Parietaria (Pellitory) has scattered leaves without large stipules, and stinging hairs are absent. The ♀-perianth is 4-toothed, flask- or bell-shaped.—Pilea is a tropical genus with trimerous, zygomorphic ♀-flowers, the posterior perianth-leaf being much larger than the two others, and more or less hood shaped.—The flower of Forskohlea is the most reduced; the ♂-flower has only one stamen, and the ♀-as well as the ♂-flowers have a one-sided, tongue like perianth (?). Pouzolzia.