THALLOPHYTES 



47 



u;ives rise to erect branches, whose tips become sporangia 

 that produce vast numbers of spores that are scattered by 

 currents of air (Figs. 26 and 27). These spore-bearing 

 branches are well called sporophores (" spore-bearers "). 



Under other conditions, two neighboring -mycelia form 

 special branches that come together in pairs, tip to tip (Fig. 

 28). Each tip is cut off frpm the rest of the body by a wall, 

 and the protoplasts of the two cells thus formed fuse, and a 

 heavy-walled oospore is the result. This means thajt each 

 tip-cell is a gametangium, and 

 that the fusing protoplasts are 

 gametes. The gametes and the 

 gametangia usually look alike 

 (Fig. 28, B) and behave alike, 

 but it is found that the mycelia 

 are sexually different. In some 

 cases the gametangia differ in 

 size (Fig. 28, C), so that a sexual 

 difference is evident. Although 

 one mycelium looks very much 

 like another, the formation of 

 oospores will not take place 

 unless sexually different mycelia 

 are brought together. For this 

 reason the mycelium of molds 

 may be grown indefinitely with- 

 out producing oospores. 



The four things to observe, 

 therefore, in the study of a true 



fungus, are the mycelium, the haustoria, the sporophores, 

 and the sexual apparatus. A comparison of the mold with 

 some other Fungi will illustrate how these four things vary. 



Downy mildews. There is a group of Fungi called the 

 " downy mildews/' which attack a great many plants, pro- 

 ducing such diseases as potato rot, grape mildew, and com- 



Fio. 28. Sexual reproduction of 

 Mucor: A, the sexual branches in 

 contact ; B, the two sex-organs 

 (gametangia) cut off by walls ; C, 

 the two pairing sexual branches 

 and their gametangia unequal in 

 size ; D, the oospore formed by 

 the fusion of the protoplasts of 

 the two gametangia. 



