PARASITIC WEEDS 



93 



host plants on which they live, as they drain the hosts of so 

 much food that the latter are necessarily much enfeebled. 

 Partial parasites usually do less harm, because their demands 

 are not so heavy, but they can do much injury if they attack 

 weakly hosts. 



Among farm weeds there are a few species that live 

 parasitically, some of which 

 are of real economic impor- 

 tance, while the rest are more 

 interesting than dangerous. 

 Both total and partial para- 

 sites are represented, dodder 

 (Cuscuta spp.) and broomrape 

 (Orobanche spp.*) being the 

 chief total parasites, and 

 yellow rattle (Rhinanthus 

 crista-galli] and red bartsia 

 (Bartsia odontites) the most 

 common partial parasites. In 

 addition to these, eye-bright 

 (Euphrasia spp.\ cow-wheat 

 (Melampyrum spp.) and red 

 rattle (Pedicularis spp.) are 

 sometimes found in grass- 

 land and are probably parti- 

 ally parasitic on the roots of 

 grasses, but they are not 

 known to do much damage. 



DODDER (Cuscuta spp.\ 

 (Fig. 26), Nat. Order Con- 

 volvulacece. Dodder is fre- 

 quently seen amongst clover 

 and gorse, appearing as long, 

 slender red threads which 

 interlace the stems and 

 branches. The threads are 

 not rooted in the ground and they bear no leaves, but are 

 studded in the summer with clusters of small white flowers 

 which are followed by four-seeded capsules or seed vessels. 

 Several species of dodder are scattered over the world, each 

 species having one or more host plants on which it usually 

 lives. Among others the clover dodder (Cuscuta trifolii) 

 patronises clover and other leguminous plants ; flax dodder 



FIG. 26. DODDER (Cuscuta trifolii), 

 parasitic on Clover. 



