FUNGI AND PLANT DISEASE 



417 



included here. The so-called root parasites (fig. 254) attach 

 themselves to the roots or underground stems of their host plants. 

 Some obtain in this way their entire food supply, as in the case 

 of beech drops, squaw root, and broom rape, all completely 

 lacking in chlorophyll. Others depend upon their host plants 

 for water and mineral salts only; examples of the latter group 

 are seen in the green, herbaceous cow wheat, yellow rattle, 

 eyebright, and painted cup. Stem parasites attach themselves 



Fig. 255. — Dodder is a stem parasite, forcing an absorbing haustorium into 

 the conducting channels of its host upon which it depends for water and food. 

 Parasite can be seen in lower half of illustration, host plant in upper half. 



to the above ground parts of their hosts. Here again there are 

 members which are completely dependent upon the host for 

 foods, the best known of which are the common dodders of 

 clover and alfalfa (fig. 255). These are twining plants which 

 encircle the host plant stems, sending into their tissues haustoria, 

 or root-like absorbing organs which absorb foods for the parasite. 

 Dodder seeds overwinter in the ground, or may be found con- 

 taminating clover or alfalfa seed, which is its chief method of 

 dispersal. The dwarf mistletoes are colorless stem parasites of 

 many coniferous trees, especially those of the arid and mountain 



