THE SPORE-SAC FUNGI 499 



germinates by forming directly a i)scudo-stem which bears a sporan- 

 gium soon filled with dust-spores. The formation of zygospores at 

 once suggests kinship with alga? like Spirogyra, and it is believed 

 that molds of the type here shown may have evolved from Chloro- 

 phyceae similar to the "pond-scums." The zygomycetes are fungi 

 which produce zygospores. 



181. The water-mold fungi (Class Oomycetes) are typified 

 by a small group closely resembling alga^ because of their aquatic 

 habits. These water-molds, as they are called, are well represented 

 by species of Saprolegnia (Fig. 327) which grow commonly upon 

 dead insects (A) or succulent plant-fragments decaying in water; 

 or, in some cases, as parasites upon fish. As with the bread-molds 

 the feeding mycelium is unicellular. Projecting hyphse fonn ter- 

 minal sporangia from w'hich swarm-spores emerge (B, C). Female 

 gametangia are formed b}^ swellings on certain hj^ihse, the protoplast 

 becoming transformed into one or more spherical gametes (E). 

 Meanwhile, male gametangia develop either from the same hypha 

 or from hyphae near by, as club-shaped organs which grow toward 

 the female gametangium and send a projection through its wall 

 to the gametes within. After fertilization the female gametes be- 

 come resistant oospores. These eventually are set free and ger- 

 minate by sending out a sporangial hypha in which swarm-spores 

 are developed. In some of the water-molds degeneration of the 

 sexual reproductive organs has gone so far that, although male 

 gametangia are developed no fertilization takes place and oospores 

 form non-sexually. In extreme cases oospores are formed although 

 no vestige of a male gametangium appears. The life-history of the 

 water-molds is essentially similar to that of other oomycetes which 

 are parasitic upon land-plants. All of these fungi, to judge from 

 their methods of reproduction are more nearly akin to algae of the 

 type represented by Coleochsete than to any other we have studied, 

 although important cUfferences show that the kinship is rather 

 remote. Oomycetes are fungi which produce oospores. 



182. The spore-sac fungi (Class Ascomycetes) may be 



illustrated sufficiently for our purpose by the ''mildews." 

 These, typified by the "powdery mildew" Erysibe, are 

 parasitic upon the aerial parts of higher land-plants. As 

 shown in Fig. 328 the feeding hyphas creep over the surface 

 of a leaf, and at short intervals send out projections into the 

 host. These projections by means of which the food is ab- 

 sorbed, are called haustoria. ^ Another peculiarity distinguish- 

 ing these vegetative hyphse from any previously studied is 

 the presence of frequent cross-partitions. Dust-spores are 



^ Haus-tor'i-um < L. huustor, a drinker. 



