102 



THE FUNGI WHICH CAUSE PLANT DISEASE 



Asexual spores are either in sporangia or are borne as conidia. 

 The sporangium is usually with a columella. The spore-bearing 

 stalks exhibit the widest diversity in shape and form of branch- 

 ing, Fig. 69. 



Sexual spores (zygotes) are produced through the union of two 

 like gametangia. (Fig. 70.) Though the cytology of zygote 

 formation has not been completely studied it seems clear that the 

 fertilization is multi-nucleate ^^■' as in Albugo bliti and that the 

 two uniting elements are coenogametes. 



Key to Orders of Zygomycetes 



Asexual spores borne in sporangia which 

 in some genera are reduced to 

 corddia-like bodies 



Asexual spores true conidia borne singly 

 at the apex of the conidiophores .... 



Mucorales, p. 102. 

 Entomophthorales, p. 107 



Mucorales (p. 66) 



This order is comprised mainly of saprophytes, about twenty 

 genera and one hundred fifty species; but includes a few forms 



which prey upon vegetation in a very 

 low ebb of life, as cells of ripe fruit, 

 tubers, etc., and a few species which 

 are of especial interest as they grow 

 upon other fungi. The sporangial 

 stage is exceedingly common; the 

 zygosporic much less so, verj^ rare in 

 the case of some species. Blakeslee ^-° 

 has shown that in some species, 

 though the two uniting sexual organs 

 FiQ. 71.— Phycomycetes showing are to all appearances alike, the plants 



zygosporic lines at regions of . ^^ , i ^^n^ 



contact between + and — are in reality dicEcious; that a branch 



strains. After Blakeslee. r i ^ , , 



irom one plant cannot produce sexual 

 organs that will unite with other sexual organs produced upon 

 the same plant. Moreover, there appears to be a differentia- 

 tion of sex in that one plant, which may provisionally be re- 



