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. europea, 
but several aliens; N.O. Convolvulacee (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 
m from any contact with the soil. It 
Fic. 42.—Cuscuta (Dov- lives on its host alone, leafless and 
DER)GRowINGoNHor. rootless, strangling its benefactor 
pisteerhinia sucker (haus with a multitude of brown threads. 
toria). 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. In 
spite of its colour, the plant is a total parasite, all its food 
being drawn from the host. Chlorophyll is present, but it 
