THALLOPHYTA: FUNGI 



137 



mature and are liberated during the following spring. Like the conidia, 

 they directly infect new hosts. 



Venturia. Venturia inaequalis is the cause of an apple disease known 

 as apple scab. It affects chiefly the leaves and fruits, producing brown 

 spots that become scaly as a result of cork formation. The mycelium 

 grows between the cuticle and the epidermis. It forms large numbers of 



Fig. 112. Galls produced on cherry twigs by the black-knot fungus, Plowrightia morbosa, 

 natural size. 



conidiophores that break through to the surface (Fig. 114A). Conidia, 

 abstricted from their tips, spread the fungus during the summer to other 

 apple trees. In the autumn, after the infected leaves fall to the ground, 

 the mycelium becomes saprophytic and produces perithecia in the follow- 

 ing spring (Fig. 1 145). These appear on the lower side deeply embedded 

 within the leaf tissues. Sex organs are produced, but the ascocarp begins 

 to develop before fertilization has occurred. The ascogonium is long, 

 coiled, and multinucleate. It has a trichogyne with which the anther- 

 idium comes in contact. Following fertilization, the ascogonium gives 

 rise to ascogenous hyphae from which asci are developed as a result of 

 hook formation at their tips. The ascocarps (perithecia) are dark brown 

 and flask-shaped when mature, discharging the ascospores forcibly. 



