Hydnora

Article

December 3, 2021

Hydnora, in older Czech literature also pride, is a genus of flowering plants from the family Aristolochiaceae. They are plants of a very unusual appearance, as they are underground non-green parasites of roots of spruces and woody plants from the legumes and birch. Under the ground, they grow with an extensive system of bumpy organs resembling roots, but they are probably more heavily modified rhizomes. They penetrate the tissues of their hosts. Their flowers and possibly fruits are the only at least partly above-ground part of the plant. The petals are very fleshy and produce unpleasant-smelling substances that attract some beetles or bipeds. The fruits filled with small seeds are about 10 cm large berries with tasty and sweet flesh. Fruit-eating mammals contribute to the spread of seeds. The genus Hydnora includes about 8 species found in arid regions of Africa, Madagascar and the southern Arabian Peninsula.

Description

The genus Hydnora includes morphologically heavily modified and reduced non-green holoparasitic perennial herbs. They completely lack leaves or organs derived from them, which is very rare in flowering plants. Because they do not need to be photosynthetic (they are heterotrophic) and parasitize on the roots of their hosts, hydropods are purely underground plants, only their flowers and fruits are partly above ground. However, the species Hydnora triceps also blooms underground.

Vegetative organs

Underground, hydropods grow through an extensive branching system of more or less horizontal root-like organs of unclear nature - sometimes considered more as rhizomes, other times more as roots. These organs are fleshy, several cm to 1 dm thick, conspicuously square (four to six angles in cross section) or rounded, sometimes flattened. On the surface they are markedly warty, in species with angular underground organs, these bumps are arranged in rows just at the edges, otherwise they are scattered on the surface randomly. Although the bumps also contain meristematic cells, they are usually dormant and do not grow further. However, some develop into new branches and in the younger parts of the plant also into flower buds. In contact with the root of the host plant, haustoria penetrate the bumps and penetrate the host tissues. Haustoria are made up of elongated parenchymal cells and vascular bundles. The vessels of the haustorias can connect directly to the vessels of the host. The homology between the underground organs of hydnor and the vegetative organs of other plants is considerably obscured by their parasitic way of life. Unlike typical rhizomes, hydnor underground organs lack clear articulation of any leaf-like organs (eg scales found on rhizomes or above-ground stems of many other holoparasites). They also lack any trichomes, vents or lenticels. At the tops, on the other hand, they have a formation resembling a root cap, a structure typical of a root growth apex. However, in the case of hydnor, the nature of this tissue is not entirely clear, it may also be a derivative of the stem periderm produced by the meristem of the growing apex. On the other hand, the underground organs lack hydnor and many features of the root (they do not have root hair or endodermis) and the arrangement of vascular bundles corresponds to the stem: collateral vascular bundles are in a circle and surround the central marrow. The parenchymal cells contain a large concentration of tannins, which give the plant a bitter and astringent taste. In the cut, the plant drips a sticky liquid. The surface of the vegetative organs, including the growth apex, is covered with a cork layer of suberinized cells. Secondary thickening is ensured by both phelogen and cambium, which, however, do not form a complete circle in cross section - it is about

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