ASCOMYCETES 269 



According to Clinton, the conidia are probably unable to retain 

 their vitality for a considerable period of time, and therefore may 

 not be of great consequence in initiating the disease the following 

 season. Some believe, however, that the scabby spots upon old 

 fruits remain living, at least so far as the mycelium is concerned, 

 and that new conidia may be produced the following spring. 

 They would believe that this is particularly true when the fungus 

 has attacked young twigs, and that therefore it is in favorable con- 

 dition for early infection. Nevertheless, the fungus has not been 

 found constantly or abundantly upon young twigs and it is quite 

 probable that twig infections are less common than is supposed. 

 In that case, the constant reappearance of the disease may be 

 more generally due to the development of the perfect stage 

 during the winter. 



So far as the development of the perithecial form has been 

 followed in this country, it is believed that the first evidences of 

 the perithecium in the case of the apple scab are found in October 

 and later, the perithecia reaching maturity by April perhaps. At 

 any rate, mature ascospores have been found during April and 

 May, and the perithecia disappear by the following month. The 

 perithecia are usually found on the under surfaces of the leaves, 

 and Clinton believes that less conspicuous scabby spots develop 

 the perfect stage most freely. The studies which have been made 

 of the perithecia in artificial cultures, strengthened by the obser- 

 vations in the open, seem to indicate beyond any question the 

 relationships of these two forms. The perithecia are somewhat 

 imbedded in the tissues of the leaf, are spherical or nearly 

 spherical in form (Fig. 121), 90 to 150/4 in diameter, and at 

 maturity slightly beaked, these beaks being sometimes protected 

 by half a dozen or more bristles. The perithecial wall is made up 

 of cells more or less polygonal in outline. The asci are clavate to 

 objong or slightly curved, 55-75 X 6-i2/x. They are numerous in 

 the perithecia and so far as noted there are no paraphyses. The 

 spores are eight, becoming two-celled, one of which is larger than 

 the other. The spores are olive-brown in color, 11-15 X 5-7 /*. 



The histological development of the perithecium has not been 

 followed. The ascospores germinate readily in water, and some- 

 times true appressoria are produced, as stated in the case of the 



