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A TEXTBOOK OF THEORETICAL BOTANY 



which show various degrees of complexity in different genera. Their shapes 

 are quite characteristic and are chiefly used as a basis of the separation of 

 the genera. 



The hfe-cycle of Sphaerotheca humuli is represented in the following 

 diagram (Fig. 247). 



Blnucleote 



Antheridlum 



Ascospores 



Conidiospores 



^Sphaerotheca 

 Mycdium 



Fig. 247. — Life-cycle oi Sphaerotheca hiimuli. 



In the classification of the Powdery Mildews we have a clear and simple 

 example of the principles of plant classification, and it may be worth while 

 to mention it briefly here. The genera fall naturally into two groups ; firstly 

 those with only one ascus in each perithecium, and secondly those in which 

 more than one ascus is produced. In the former section there are two genera : 

 Sphaerotheca, in which the appendages are long and hyphal-like, and Podo- 

 sphaera, in which the ends of the appendages are repeatedly and dichotomously 

 branched. In the second group are four common genera : Erysiphe, in which 

 the appendages are like those of Sphaerotheca ; Microsphaera, in which they 

 closely resemble Podosphaera ; Uncinula, in which the tips of the appendages 

 curl inwards ; and, finally, Phyllacthua, in which they are straight and stiff 

 with a swelling at the base. The following diagram brings out these points 

 clearly (Fig. 248). 



Erysiphe polygoni (The Pea Mildew) 



This species causes a Mildew of Peas and also of Swedes and Turnips. 

 In the former case it may attack the pods as well as the leaves. Swedes 

 and Turnips are often attacked in a dry summer to such an extent that the 

 leaves are turned completely white by the mycelium. 



The Fungus spreads over the surface of the host plant, sending down 

 fine haustoria into the epidermal cells where they spread out forming 

 absorptive organs. During the summer months reproduction is by means 

 of conidiophores which abstrict conidiospores in chains (Fig. 249). These 

 are entirely similar to those in Sphaerotheca both in their development and 



