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. europoea, 

 but several aliens ; N.O. Convolvulacese (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 circunmutation (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. In 

 spite of its colour, the plant is a total parasite, all its food 

 being drawn from the host. Chlorophyll is present, but it 



.a 



FIG. 42. Cuscuta (DOD- 

 DER) GROWING ON HOP. 



a, absorbing suckers (haus- 

 toria). 



