TYPES OF CRYPTOGAMS; THALLOPHYTES 



259 



After about twenty-four hours examine another portion of the 

 mould from the surface of the liquid and study the more fully 

 developed mycelium. Sketch. 



309. Zygospores. Besides 

 the spores just studied, zy go- 

 spores are formed by conju- 

 gation of the hyphse of the 

 black moulds. It is not very 

 easy to find these in process 

 of formation, but the student 

 may be able to gather from 

 Fig. 190 the nature of the 

 process by which they are 

 formed, a process which can- 

 not fail to remind him of the 

 conjugation of pond-scum. 



THE STUDY OF WHEAT 



RUST (PUCCINIA 



GKAMTNB) 



310. Occurrence. Wheat 

 rust is common on cultivated 

 wheat and other grains, and 

 also on many wild and culti- 

 vated forage grasses. In fact, 

 this or similar rusts occur on 

 a very large number of grasses, 

 and many species of such rusts 

 are recognked. A rust may 

 have one, two, or three kinds 

 of spores, and when three occur one is known as the cluster-cup stage 

 and the others as red rust and black rust, according to the usual 

 approximate color of the spores. The rust called Puccinia graminis 

 growing on wheat has its cluster-cup stage on the leaves of barberry 

 in June. The spores from the cluster-cups are carried by the wind 

 to the wheat, where they germinate and in a few days produce the 



FIG. 190. Formation of Zygospores in a 



Mould (Mucor Mucedo). 

 1, threads in contact previous to conjuga- 

 tion ; 2, cutting off of the conjugating 

 cells, a, from the threads, b ; 3, a later 

 stage of the process ; 4, ripe zygospore ; 5, 

 germination of a zygospore and formation 

 of a spore-case. (1-4 magnified 225 diam- 

 eters, 5 magnified about 60 diameters.) 



