. British plants : their biology and ecology. Plants; Plant ecology. 128 BRITISH PLANTS to one definite species of host, and are never found on any other. Others are not so particular. The drain upon the host is generally so great that it prematurely perishes ; fields of clover are sometimes ruined by the parasite. (3) Cuscuta—dodder; one native species, C. europcea, but several aliens ; Convolvulaceae (Fig. 42). The life-history of this climbing parasite is very interesting. The seed germinates on the ground, thrusting out a short anchoring root. Then a long attenuated filament grows out
. British plants : their biology and ecology. Plants; Plant ecology. 128 BRITISH PLANTS to one definite species of host, and are never found on any other. Others are not so particular. The drain upon the host is generally so great that it prematurely perishes ; fields of clover are sometimes ruined by the parasite. (3) Cuscuta—dodder; one native species, C. europcea, but several aliens ; Convolvulaceae (Fig. 42). The life-history of this climbing parasite is very interesting. The seed germinates on the ground, thrusting out a short anchoring root. Then a long attenuated filament grows out, devoid of leaves. This is the primary shoot. It is colourless, and its growing apex is sensitive and displays active circumnutation (p. 116). It grows rapidly, form- ing loops along the ground until the food-material stored up in the seed is exhausted. If by this time the circumnutating tip does not meet a suitable host it dies. If it does, the sensitive thread twines round its stem, putting out suckers which penetrate the tissues of the host at every point of contact. Having established itself securely, the rear part dies away, and for the rest of its life the dodder is free from any contact with the soil. It lives on its host alone, leafless and rootless, strangling its benefactor with a multitude of brown threads. These in the course of time bear clusters of white, bell - shaped flowers, covering the host with flowers not its own. After setting its seed the parasite perishes. The dodder is found on a variety of plants : heath, gorse, thyme, wood-sage, etc. The Mistletoe (Viscum album, Fig. 43) is a parasite of peculiar interest. It is a green leafy plant, and yet it possesses a highly elaborate and most effective mechanism for the absorption of nutriment from its host. It is a shrubby evergreen, and in the course of time the base of its stem becomes so embedded in the tissues of the host that host and parasite appear to form one plant. Id spite of its colour, the plant i
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