THE FUNGI WHICH CAUSE PLANT DISEASE 103 



garded as the male, unites freely with another plant, provisionally 

 the female, but this male plant refuses to unite with any other 

 plant which is capable of uniting with the female and all plants 

 that can unite with the male refuse to unite with the females. In 

 some species the plants of one sex show a more luxuriant vegeta- 

 tive growth than do plants of the other sex. 



Key to Families of Mucorales. 



Asexual spores in typical sporangia, although 

 in some genera few-spored 

 Sporangium with columella; zygospores 

 naked or thinly covered with out- 

 growths of the suspensor 1. Mucoracese, p. 103. 



Sporangium without a columella; zygo- 

 spores closely covered by hyphae ... 2. Mortierellaces. 

 Asexual sporangia monosporic and conidia- 

 hke, sometimes accompanied by larger 

 polysporic sporangia 

 Sporangia of two kinds, polysporic and 



monosporic 3. Choanephoraceae, p. 106. 



Sporangia all monosporic; parasitic on 



other genera of Mucorales 4. Chsetocladiaces. 



Sporangia simulating chains of conidia. . 5. Piptocephalidaceae. 



Of these families the second and fifth are pure saprophytes, 

 while the fourth is parasitic upon other members of the order. 



Mucoracese 



Mycelial threads all alike or of two kinds, one aerial, the other 

 buried in the substratum, ccenocytic during growth but septate 

 at maturity; reproduction by asexual spores borne in sporangia 

 and by zygospores formed by the union of equal gametes; spor- 

 angiophores, simple or branched; sporangia variable, typically 

 with a columella, and many spores but in some genera some of 

 the sporangia are few-spored and without columellas; zygospores 

 variable, smooth or spiny, borne on short branches of the myce- 

 lium. 



