SAPROPHYTISM AND SYMBIOSIS 



763 



species that forms galls on legume roots (p. 787), also lives saprophyti- 

 cally in the soil, and thus is a facultative form. 



The mildews, rusts, and smuts are representative parasitic fungi, 

 most of which are deleterious to their host plants (fig. 180), some species 

 producing conspicuous galls in various organs. Some of the Poly- 

 poraceae (as the bracket fungi) are harmful parasites on trees. The 

 hyphae of parasitic fungi 

 are thought to be more 

 specialized than are those 

 of saprophytic forms, hav- 

 ing greater power of pene- 

 tration into woody and 

 mechanical tissues, and it 

 is likely also that their 

 absorptive efficiency is 

 greater. The penetrative 

 power of the hyphae is 

 due in large part to the 

 substances which they se- 

 crete, particularly various 

 enzyms, such as the wood- 

 destroying enzyms of the 

 tree-inhabiting fungi. 

 Wounds of various kinds 

 greatly facilitate the inva- 



FIGS. 1079, 1080. Haustoria of parasitic fungi. 

 1079. a hyphal filament of Albugo Candida from which 

 there originate small spherical haustoria that pene- 

 trate the parenchyma cells of the host plant, Lepi- 

 diunt sativum ; 1080, a hyphal filament of Peronospora 

 calotheca from which there originate richly branch- 

 ing haustoria that penetrate all parts of the paren- 

 chyma cells of the host plant, Asperula odorata; both 

 figures highly magnified. From DEBARY. 



sion of plant organs by 

 parasites. While ordinary 

 hyphae often invade the 

 living host cells, in many cases they creep along the outer surface (as in 

 various mildews) or penetrate between the cells, special branches known 

 as haustoria piercing the walls (especially through the pits) and absorbing 

 the contents of the lumina; surface forms are known as ectoparasites, 

 while internal forms are termed endoparasites. In Albugo the haustoria 

 are knoblike processes (figs. 1079, 158) and in Peronospora the hausto- 

 rial branch may divide into a number of finger-like absorptive organs 

 (fig. 1080). The spores of most parasitic fungi, even of such obligate 

 forms as the rusts, germinate somewhat readily in a number of media, 

 thus differing from the seeds of the higher holoparasites; oddly enough, 

 the spores of many saprophytic fungi germinate much less readily. 



