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PLANT MORPHOLOGY 



ascogonium, which then gives rise to ascogenous hyphae. As mPyronema, 

 the asci arise as a result of hook formation at the tips of the ascogenous 

 hyphae. Their development occurs in the typical manner. 



' Plowrightia. This is another parasitic genus, its best-known species, 

 Plowrightia morbosa, causing a destructive disease of the plum and cherry 

 known as black knot. The mycelium passes the winter under the bark of 



Fig. 



B C 



111. Claviceps purpurea. A, stalked stromata arising from a sclerotium, X4; B, 



longitudinal section through a stroma, showing the embedded perithecia, X30; C, a 

 perithecium with young asci and paraphyses, X 250. 



a branch or twig. In the spring it breaks out on the surface to form an 

 elongated gall or knot consisting of both mycelium and hypertrophied host 

 tissue (Fig. 112). Leaves and fruits are not attacked. The elongated 

 knots, often reaching a length of 12 cm. or more, are developed mostly on 

 one side of the stem, which becomes more or less deformed. In early 

 summer the mycelium within the knot gives rise to innumerable short 

 conidiophores that form a velvety layer on the surface. The conidia, 

 distributed by the wind, spread the fungus to other hosts. Later in the 

 season conidium formation ceases and the knot becomes hard and black, 

 forming a stroma in which hundreds of perithecia appear (Fig. 113). 

 These are small flask-shaped organs, embedded in the stroma, and lined 

 with a hymenium consisting of asci and paraphyses. The ascospores 



