CLIMBERS, PARASITES, SAPROPHYTES. 203 



them can fix the free nitrogen of the air, but for their carbon 

 all chlorophyll-less plants appear to be dependent on the 

 organic substances manufactured by plants which have 

 chlorophyll. 



236. Total Parasites. Dodder, Broomrape, and Tooth- 

 wort are British examples of flowering plants which are 

 practically total parasites. 



The Dodder (Cuscuta) belongs to the Convolvulus order. 

 The commonest British species (0. epithymum) grows on G-orse, 

 Ling, Thyme, and Clover. The seed, which contains a thread- 

 like embryo, germinates in spring, sending a short radicle 

 into the soil, while the shoot rapidly elongates and sweeps 

 round in widening circles in the air. If it meets with a suit- 

 able plant (the plant on which a parasite grows is called, 

 rather ironically, its "host"), the Dodder stem, which is 

 endowed with tendril-like sensitiveness to contact, twines 

 around the host-plant and sends into it root-like organs 

 (termed haustoria, or " suckers ") which, by means of ferments, 

 eat as well as push their way to the bundles of the host. 



The wood- vessels and sieve-tubes of these organs fuse with 

 the wood-vessels and sieve-tubes of the host, and thus the 

 Dodder draws from the host its supplies of organic food as 

 well as water and salts. Meanwhile the radicle of the Dodder 

 dies off. The thin reddish stem of the parasite branches 

 copiously, bearing small scales (instead of ordinary leaves) 

 and numerous clusters of flowers. If the Dodder seedling 

 does not happen to reach a suitable host-plant it soon dies. 



The Broomrape and the Toothwort are root-parasites, 

 i.e. their earth-roots bear, at the ends of the rootlets, suckers 

 which apply themselves to the roots of the host-plant and 

 send in sucking organs like those of Dodder. The commonest 

 British species of Broomrape are the Greater Broomrape 

 (0. major), which grows on the roots of shrubby Papilionaceae 

 (Broom, G-orse, etc.), and the Lesser Broomrape (0. minor), 

 which grows on Clover and various other plants. 



The Toothwort (Lathraea squamaria) has a creeping 

 underground stem which branches and bears crowded fleshy 

 leaves, while the erect shoots bear thin scaly leaves and end 

 in a raceme of flowers. The leaves covering the creeping 

 underground branches are hollow and are beset internally 



