^70 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



used for planting are liable to reproduce the disease next year. Sexual reproduc- 

 tion is as yet not certainly known to occur in the life-history of the Potato-disease 

 Fungus Phytophthora omnivora and Pythium de Bm-yanum attack and destroy 

 many young seedlings, causing them to "damp off". Various species of Peronospora 

 are known which attack large numbers of cultivated plants. P. parasxUca works 

 havoc amongst the Cruciferse; P. viticola ( = Plasmopara viticola, fig. 381) has 

 been referred to as a deadly disease on the Vine; P. Vicim on various leguminous 



Fig. 381.— The False Vine-mildew, Peronospora viticoUi. 



> A bunch of grapes attacked by the False Vine-mildew. 2 Spores or conidia on branched hyphse projecting from a stoma of 

 a Vine-leajt. « Fertihzation. * A single oonidium. = Swarm-spores arising within the conidium. « A single swarm-spore. 

 1 nat. size ; 2 X 80 ; »-6 x 350 ; = x 380. (>-« after De Bary.) 



crops; P. Hyoscyami on Tobacco-plants, besides which there are many others. The 

 effects of Cystopus candidus have already been mentioned (p. 525); it is common 

 upon cruciferous plants. 



About 100 species of Peronosporese have been distinguished. 



Saprolegniacece. — Are all aquatic and for the most part saprophytic; a few are 

 parasitic on fish. In structure they much resemble Peronosporese. Swarm-spores 

 are liberated in large numbers from the enlarged ends of the hyphse (fig. 380). 

 Sexual organs arise much as in Peronospore^, but although the antheridia develop 

 fertilizing tubes in several instances, no case has been observed in which an actual 

 transfer of spermatoplasm occurs. As a rule several or many egg-cells are produced 



