THE BREAD MOLD 59 



life. Saproph^'tic plants like the mold, and parasitic ones 

 Uke many of the other fungi, are Ukely to produce great 

 nimibers of spores. This habit enables them to miiltiply 

 their numbers quickly when food of the sort that they need is 

 plentiful ; and it gives them a much better chance of living 

 over the unfavorable periods that are bound to come to such 

 plants, when the food that they need is not to be had. On 

 the other hand, plants which, Uke Spirogyra, live under 

 conditions that are likely always to be much the same, at 

 least at the same season of the yea.T, have much less need of 

 a means of rapid multiplication at special times. This is 

 why Spirogyra succeeds as well as it does without producing 

 any cells that correspond to the spores of the mold. 



87. Relatives of the Bread Mold. — Rhizopus is one of a group of 

 fungi which are commonly called the black molds. Other members of 

 this group are Mutor, whose species are common, especially on soil ; 

 PUobolus, which grows on dung ; Phycomyces, living on old bones and 

 other oily organic matter ; and Sporodinia, found on decaying mush- 

 rooms. All the black molds are characterized by a. one-celled body 

 made up of branching threads and by a method of sexual reproduction 

 closely similar in each case to that of Rhizopus. In most of the black 

 molds the two gametes of a pair are of the same size, instead of being 

 different in size as are those of Rhizopus. In Sporodinia and in some 

 other forms, both gametes are borne on branches of the same plant ; 

 but in most species they are borne, as in the bread mold, on separate 

 plants belonging to two different strains. All the black molds pro- 

 duce large numbers of asexual spores, but these spores are produced 

 in ver^• different ways in different species. Most of the black molds 

 are saprophytes, but a few are parasitic upon other fungi. 



