288 



PART IV. CLASSIFICATION. 



which, sexual plants are developed. In other species (e.g. Cystopus candidus) 

 the contents of the oospore are set free as a number of zoospores. In yet 

 other species (e.g. Pythium de-Baryanum, Pythium Artotrogus, Peronospora 

 Valerianellce), the oospore directly gives rise to a sexual plant. 



Pythium can be obtained by sowing cress in a pan of earth. When the 

 seedlings spring up, they should be well watered and be kept covered with 

 a bell-jar so as to keep them moist. Some of them will be seen to fall 



owing to the failure of the 

 stem just above the surface 

 of the soil ; these are infected 

 with Pythium. If an in- 

 fected stem be kept in a drop 

 of water on a slide for a day 

 or so, and be examined from 

 time to time, hyphse bearing 

 the reproductive organs of the 

 Fungus will be seen to be de- 

 veloped at the surface. 



In the genus Peronospora, 

 which is represented by 

 many species (P. parasitica 

 on Capsella, P. calotheca on 

 Rubiacese, etc.), only one 

 sporangium is borne by each 

 branch of the sporophore 

 which protrudes through a 

 stoma. In Phytophthora 

 the sporangia are displaced 

 laterally by branches which 

 arise from the hyphte bearing 

 the sporangia, at their points 

 of origin. To this genus 

 belongs P. infestans, which 

 produces the potato-disease. 

 The tissues of the host un- 

 dergo decomposition in the 

 infected parts and turn 

 black : the mycelium of the 

 Fungus extends from the 

 circumference of these spots, 

 and throws up sporophores 

 through the stomata (Fig. 



FIG. 168. .4 Surface-view of the epidermis of a 

 Potato-leaf with the sporophores of Phytophthora 

 infestant projecting out of the stomata ( x 90). B A 

 ripe sporangium. C Another undergoing division. 



D A zoospore. ( x 540 : after Strasburger.) 168). The sporangia of the 



parasite are carried by the 



wind to healthy plants and infect them : the zoospores also penetrate 

 through the soil to the tubers, and the mycelium which is developed from 

 them extends into the young Potato-plant which grows from the tuber. 

 No sexual reproductive organs have been observed in this Fungus as yet. 



