THE FUNGI WHICH CAUSE PLANT DISEASE 115 



reproductive structures in the form of conidia. These may be 

 borne singly or in rows on simple or branched conidiophores. 



The conidiophores may be single or variously grouped in columns 

 or layers. Figs. 352, 378, 382. In some instances they are very 



Fig. 75. — Sphsrotheca castagnei. Fertilization and de- 

 velopment of the perithecium. Og= oogonium, an= 

 antheridium, st= stalk-cell. 6 as the ascogonium 

 derived from the oogonium. After Harper. 



short, innate; again they are long, loose or floccosc. They may 

 emerge through stomata singly or in tufts or they may form sporo- 

 genous cushions below the epidermis or again they may be borne 

 inside of a hollow structure, the pycnidium, which covers them. 

 Chlamydospores are also found. 

 One or several distinct types of 

 sporification may belong to one 

 species of Ascomycete. These dif- 

 ferent forms of spores may appear 

 simultaneously on the same myce- 

 lium or they may follow in definite 

 succession regulated by the changes 

 in environment, or again one or 

 more of the spore forms belonging to the Hfe history of the fun- 

 gus may be omitted for long intervals to appear only as the 

 result of stimuli of which little is yet known. 



The conidia and chlamydospores are asexual spores. Sexuality 



Fig. 76. — Boudiera. Six sets of 

 sexual organs. After Claussen. 



