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TEXTBOOK OF BOTANY 



Fig. 241. Reproductive structures of a powdery mildew: A, conidia formed 

 during the summer months; B, an ascocarp, which hves through the winter and 

 within which the asci and ascospores develop. They can be forced out of the 

 ascocarp by a slight pressure as indicated in the drawing. B from C. J. Chamber- 

 lain, Elements of Plant Science, McGraw-Hill Book Company, Inc. 



in chains by successive abstrictions (Fig. 240). Species of Pemcillium 

 have blue or green spores. The related mold, Aspergillus, differs from 

 the blue mold in producing globular masses of green, yellow, or black 

 spores at the enlarged ends of upright hyphae. Spores formed by con- 

 strictions at the ends of hyphae are called conidia ( sing, conidium ) , and 

 the hyphae producing them are conidiophores. Conidia are readily car- 

 ried by air currents or by rain water; and since they are produced in 

 such enormous numbers they are present everywhere, especially during 

 the summer months. 



Powdery mildews. During summer and early autumn cobwebby 

 mycelia may be seen on the leaves of such plants as ragweed, knotweed, 

 red clover, lilac, crimson rambler, sunflowers, and willows. The hyphae 

 of these fungi also have cross walls. When in contact with a leaf, some 

 of the branches of the hyphae become haustoria or rhizoids which pene- 

 trate the epidermal cells of the host tissue. The mycelia of these fungi 

 are on the surface of the host. Other hvphae, growing erect from the sur- 

 face of the leaf, form at their tips chains of colorless conidia in a manner 

 similar to that of the blue molds noted above. The conidia, or summer 

 spores, are carried about by air currents from leaf to leaf, and many 

 plants thus become infected during the growing season. These fungi are 

 called powdery mildews because of the appearance of the conidia. 



