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animals. They muSt therefore obtain their food either as para- 

 sites, and prey upon other living plants or aninjals, or as 

 saprophytes, living upon dead organic substances. In some cases 

 a fungus may resort to both these methods ; it may be parasitic 

 in one stage of its development and saprophytic in another. 



Structure of Pkronospora Parasitica. 



Peronospora jyarasitica belongs to that division of the Fungi 

 which is known as Peronosporese, and which includes 

 a large number of forms, most of which are well known to 

 agriculturists and others as producing many of the diseases to 

 which plants are liable, such as the potato and turnip diseases 

 and many others. 



The Peronosporea? are, in the majority of cases, parasitic in 

 the interior of living plants. The numerous forms found in the 

 group differ in detail, but they ai-e all closely allied in their 

 structure and method of development. 



P. parasitica is common on many species of cruciferous plants, 

 especially the cabbage and the common Shephei'd's Purse. The 

 infected plants are easily distinguished from their fellows by 

 being covered externally with large, white, dusty-looking patches 

 which may be found clothing the stem, leaves and fruits, in many 

 cases completely hiding their normal structure, and often causing 

 the infected parts to swell up to many times their original size. 

 This increase in size, or hypertrophy as it is called, can be seen 

 exti-emely well in the seed capsules of Shepherd's Purse when 

 they become infected. 



In addition to P. parasitica another fungus is commonly found 

 forming white patches on the same plants ; and the two are 

 often associated with each other. This second fungus, Cystopus 

 candidus, can, however, be easily distinguished from P. parasitica, 

 the white patches of which have a dusty appearance, by its 

 smooth and shining surface. 



Structure of the Hyph.e. 



The white patches are composed of large numbers of spore- 

 bearing filaments which are connected below the epidermis of the 

 plant, with a large number of other filaments which penetrate in 



