THALLOPHYTA: FUNGI 



127 



Plectascales, the asci being irregularly scattered throughout the mass of 

 sterile hyphae (Fig. 101). 



4. Perisporiales 



The Perisporiales, or powdery mildews, are superficial parasites attack- 

 ing many kinds of seed plants, such as grape, lilac, willow, rose, squash, 

 bean, pea, apple, grasses, and numerous others. They number about 

 500 species. Common genera are Sphaerotheca, Erysiphe, Uncinula, 

 Podosphaera, Microsphaera, and Phyllactinia. The mycelium lives on 



Fig. 102. Erysiphe graminis growing on surface of grass leaf, showing haustoria in 

 epidermis of host and conidia in various stages of development, X500. 



the surface of the leaves, forming whitish patches. Short haustorial 

 branches are sent into the epidermal cells and through them food is 

 absorbed. During the summer the mycelium produces erect conidio- 

 phores, which give rise to chains of conidia (Fig. 102). These are very 

 abundant and result in a rapid spread of the fungus to uninfected hosts. 

 In the autumn closed ascocarps (cleistothecia) appear. They are minute, 

 spherical, dark brown or black bodies with long appendages that, in some 

 genera, are branched at the tip (Fig. 103). Inside the ascocarps are the 

 asci, each usually with eight ascospores. The ascocarps, scattered by 

 the wind, survive the winter. During the next season the ascospores pro- 

 duce new mycelia. 



The character of the appendages produced by the ascocarps is impor- 

 tant in distinguishing genera from one another. Thus in Sphaerotheca 

 and Erysiphe the tips of the appendages are undivided, while in Podo- 

 sphaera and Microsphaera they are dichotomously divided. In Uncimda 

 the tips of the appendages are hooked or curved, while in Phyllactinia 

 they are straight but the appendages are swollen at the base so as to 

 form an enlarged plate. 



