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. Vicioe on various leguminous- 



Kig. 381.— The False Vine-mildew, Peronospora viticola. 



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

 a Vine-leaf. * Fertilization. * A single conidium. 5 Swarm-spores arising within the conidium. « A single swarm-spore. 

 1 nat. size ; « x 80 ; >-' x 360 ; « x 380. {'-' after De Bary. ) 



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

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

 upon cruciferous plants. 



About 100 species of Peronosporeae have been distinguished. 



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

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

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

 Sexual organs arise much as in Peronosporeae, 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 



