FUNGI: SAC FUNGI 263 



a great variety of plants, on leaves, stems, flowers, and fruit. 

 Many of the common mildews belong here. The mycelium grows 

 on the surface of the host, forming a thin and irregular web-like 

 whitish layer, just visible to the eye. Branches called haustoria 

 penetrate to the epidermal cells, in some species even to the deeper 

 cells, and draw nutriment from the protoplasm. When very 

 young leaves and stems are affected they are often checked in 

 growth. The large number of white conidia (or conidia-spores) in 

 chains or in loose masses give a powdery appearance to the sur- 

 face of the plants affected, hence the name powdery mildews. 

 Some of the important diseases caused by the powdery mildews are 

 the gooseberry mildew; the cherry mildew, growing on cherry, 

 peach, and apple trees, especially injurious to nursery stock; the 

 rose mildew, lilac mildew, etc. 



Lilac Mildew (Microsphcera Alni). 



424. The conidial stage. This is developed in asexual repro- 

 duction. Short erect branches arise from the superficial mycelium 

 which are divided by cross walls into short cells. The branches 

 grow at the base and continue to divide into short cells, raising 

 the older cells farther and farther away from the surface. At the 

 same time the older cells swell out somewhat so that they appear 

 like chains of beads, or small barrel-shaped spores or conidia. 

 The older ones separate and fall upon the surface of the leaf or 

 stem, etc., giving a powdery appearance. These conidia are 

 carried to other plants or other parts of the same plant, there 

 spreading the disease. 



425. The ascus stage, or perfect stage. This is developed 

 after the formation of conidia as the result of a sexual process 

 ending in the formation of minute brown or blackish fruit bodies,* 

 a cellular structure formed of septate threads which envelop the 

 developing asci. These fruit bodies are often very numerous, 

 appearing as minute black specks just visible to the eye. The 

 fruit body is provided with appendages of a dark color, consisting 



* Called here a perithecium. 



