PARASITES 



103 



until the hosts die and then continue to live as saprophytes on the 

 dead insect bodies. Those fungi that are obHgate parasites and must 

 have a Hving host in order to complete their life cycles, and yet which 

 habitually or periodically spend a part of their lives in a non-parasitic 

 condition, are sometimes called tropoparasites. 



Familiar examples of holoparasitic seed plants are the various 

 species of dodder (Cuscuta). The dodders are twining plants which 

 lack chlorophyll and have a yellowish color. The seeds germinate 

 on soil, usually rather late in spring after other vegetation has 

 sprouted and young shoots of host plants are therefore available. 

 The young shoot of a dodder is a fine, yellow, thread-like structure 



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Fig. 48. — Arceuthobium Douglasii (dwarf mistletoe). Pistillate plants on Pseu- 

 dotsiiga iaxifolia. (Photograph by James R. Weir.) 



whose tip rotates as it elongates. If it does not come in contact with 

 a host plant it lives but a few weeks at most. If, howTver, it suc- 

 ceeds in finding a suitable host it grows vigorously, twining about 

 the host and at the same time penetrating it with absorbing organs, 

 called haustoria, w^iich in this case are modified adventitious roots. 

 Soon the dodder loses all connection with the soil and becomes purely 

 a holoparasite. Some species of dodder may grow on various kinds 

 of hosts while others are restricted to a single species. Sometimes 

 these parasites are very destructive to crops such as clover and flax. 

 Other important holoparasites are found in the family Oro- 

 banchaceae, a family of root parasites. The seeds of the broom-rape 

 (Orohanche) germinate only when in contact with the root of a host 

 plant. The seedling penetrates the host at once and produces in the 



