670 



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 Baryanum 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. parasitica works 

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

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



Fig. 381. The False Vine-mildew, Peronospora viticola. 

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

 Vine-leaf * Fertilization. * A single conidium. Swarm-spores arising within the conidium. 6 A single swarm-spore, 

 i nat sue; x80; -6 X 350; x380. (8- 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 



itic on fish. In structure they much resemble Peronospore^. Swarm-spores 



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



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



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



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



