ASCOMYCETES 



277 



of the perithecium have not been very carefully followed, although 

 it would appear that the formation of asci in the perithecium is pre- 

 ceded by a central mass of hyaline cells, which are displaced as 

 the asci arise. At maturity the asci are oblong-clavate, 5 5-70 x g/M. 

 Each ascus contains eight spores, frequently arranged in pairs, 

 though sometimes in oblique series. The ascospores are curved, 

 but in general resemble the conidia. They are, however, more uni- 

 form in size, being usually 12-22 x 3 J— 5/*. The asci are evanes- 

 cent, and the ascospores germinate without a period of rest. 



It is not believed that the ascus-bearing stage is at all essential 

 to the annual appearance of this disease. It has been frequently 

 shown that the conidia in mummied apples retain their vitality 

 until the following season, and it is probable that infection could 

 result from conidia produced on apples which have remained on 

 the ground throughout the winter. However, when all mummied 

 fruits in the trees as well as under the trees have been carefully 

 destroyed, the disease has been found abundantly the following 

 season. It is therefore probable that a great many infections re- 

 sult through canker spots formed during the summer. These live 

 over winter and produce conidia again during the early summer. 



Cultural relations. This fungus may be readily cultivated in 

 the laboratory on almost any of the ordinary nutrient media. 

 Apple agar is especially favorable, but, as already indicated, apple 

 corn meal agar is perhaps as good as any other medium for the 

 production of the ascosporic stage. The conidia germinate within 

 a few hours and the mycelium grows with great rapidity. The 

 mycelium is septate, and neighboring cells of different mycelia 

 readily fuse. In the tissues of the apple the hyphas are brown 

 with age. In culture, however, the hyphae are at first only 

 slightly colored. The conidia are produced in quantity in culture, 

 appearing upon an agar plate within twelve hours. Under such 

 conditions the conidia are generally borne upon numerous lateral 

 branches. Upon sterilized fruit in the laboratory the production 

 of conidia within the pustules has required, under the most favor- 

 able conditions, only forty-eight hours. It is evident therefore 

 that in the field many generations of this fungus may be pro- 

 duced within a very short time, and that its rapid spread is well 

 explained by the brief period required for spore production. 



