Urticaceae

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Urticaceae

Description

Herbs, subshrubs, shrubs or treelets, terrestrial, dioecious or monoecious, most parts of the plant with variously shaped cystoliths in the epidermis, becoming visible in dried material, sap watery. Leaves alternate or opposite; stipules free or connate; blade simple, with pinnate, subpalmate, (sub)triplinerved or uninervate venation. Inflorescences bisexual or unisexual, branched or unbranched and then forming (sub)capitate glomerules, bracteate. Staminate flowers with 3, 4 or 5 tepals; stamens 3, 4 or 5, inflexed in the bud, at anthesis suddenly and elastically bending outward; pistillode present. Pistillate flowers with 3 or 4 (or 5), free or connate tepals or perianth lacking; ovary free, unilocular, ovule 1, (sub)basally attached, stigma 1. Fruit an achene, enveloped by a slightly enlarged (sometimes fleshy) perianth, subtended by small tepals or naked; seed small, with endosperm, embryo with flat cotyledons.

Distribution

Guianas: present Neotropics: present extending into temperate regions: present
Pantropical (extending into temperate regions): approx. 40 genera and approx. 1500 species; in the Neotropics 14 genera and 300-400 species; in the Guianas 5 genera and 10 species.

Wood


Vessels diffuse, solitary and in radial multiples and irregular clusters, perforations simple, intervascular pits alternate, round or polygonal.
Rays multiseriate, uniseriate rays mostly absent or scarce, heterogeneous, cells sometimes containing druses, rhombic crystals or vitreous silica. In part of the genera with unlignified parenchyma the rays are also sometimes partly unlignified.
Parenchyma vasicentric. If apotracheal parenchyma is present, it is unlignified, occurring as islands or bands as seen in transverse section. Sometimes containing druses, raphides, styloids or vitreous silica.
Fibres with simple pits on the radial and the tangential walls, all non- septate or partly septate, storied in some genera, fibre dimorphism in some genera present: short (370-1745 μm) and very long (3400-5000 μm) fibres.
Some of the genera posses crystals in the axial parenchyma. Druses are most common, although they sometimes occur in combination with rhombic crystals. Intermediate crystal forms which can neither be properly described as a druse, nor as a rhombic type, sometimes occur in the same parenchyma strands. As a result, the presence of druses and/or rhombic crystals is of limited taxonomic value. However, presence of raphides is very important in terms of taxonomy, as they are found only in the genus Laportea.
The unlignified elements, characterizing the genera Laportea, Pilea, and Urera, are sometimes absent or faint in the early stages of secondary growth in the erect or climbing shrubs and trees. The herbaceous species show unlignified bands from the beginning of their secondary growth. With increasing girth the number as well as the proportion (as percentage of the transverse section) of these unlignified elements increases. The majority of the elements is formed by axial parenchyma and is shown as islands or bands. The most extreme state of this character is found in Laportea where continuous bands occur.
In the secondary xylem of the climbing species the rays are also unlignified, resulting in a typical pattern of islands of vessels surrounded by lignified elements. The existence of this feature might well be explained in terms of mechanical strength: it occurs in lianas, climbers, and in the tall erect herbs with stems having a small diameter. On the other hand, it is hard to see how the above could apply to plants of tree-like habit (Bonsen & ter Welle, 1984).
KEY FOR IDENTIFICATIONUnlignified elements present.2All elements lignified.3Storied structure of fibres clearly visible on tangential section, vessel diameter often over 200,um.LaporteaStoried structure of fibres on tangential section not visible, vessel diameters rarely over 200 μm.Urera25-40 vessels per sq. mm, vessel diameter 40-85 μm, only primary rays.Pilea4Multiseriate rays 3-5 cells (50-120 μm) wide.BoehmeriaMultiseriate rays 2-3 cells (15-30 μm) wide.Phenax