PHYCOMYCETES 141 



the general vegetative hyphae, and also in the production of 

 other nonsexual spores, conidia, borne upon hyphae, which 

 spores germinate by means of a germ tube, and not by the pro- 

 duction of motile spores. The antheridia are always functional, 

 and the process of fertilization is apparently exactly the same as 

 in the majority of the Peronosporales. Pythium and Pythiacystis 

 are important genera. 



VII. A DAMPING-OFF FUNGUS 

 Pythium de Baryanum Hesse 



ATKINSON, GEO. F. The Potting Bed Fungus. Cornell Univ. Agl. Exp. Sta. 



Built. 94: 234-247. //. /. 1894. 



HESSE. Pythium de Baryanum, ein Endophytischer Schmarotzer. 1874. Halle. 

 MIYAKE, K. The Fertilization of Pythium de Baryanum. Ann. Bot. 15 : 



653-667. pi. 36. 1901. 

 WARD, H. MARSHALL. Observations on the Genus Pythium (Pringsh.). 



Quart. Journ. Mic. Sci. 23: 485-519. pis. 34-36. 1883. 



Habitat relations. This species is, perhaps, from an economic 

 point of view, the most important in the order. It is very com- 

 mon in greenhouse and garden soil both in Europe and America, 

 and it is a cause of one of the various greenhouse maladies known 

 as damping-off in seedlings. The conditions which favor the de- 

 velopment of the fungus are a considerable degree of warmth, 

 abundant moisture, and weakened condition of the seedlings. It 

 is especially common when the plants are being grown in a very 

 crowded condition. While most common in the greenhouse, it 

 may affect the crop in the field as well. This fungus infests a 

 variety of seedlings, but those of the cress, cucumber, sunflower, 

 and others are particularly susceptible. White clover, several cru- 

 cifers, corn and other members of the grass family are likewise 

 included among the hosts. 



Symptoms. The effects of this fungus may be evident upon 

 the seedlings a few days after germination. The point of attack 

 is ordinarily at or near the surface of the ground. The tissues 

 of the affected region promptly lose their turgidity and usually 

 appear water soaked (Fig. 43). When the tissues collapse the 

 seedlings fall prostrate, and then the mycelium invades the re- 

 mainder of the plant, if the latter is kept moist by contact with 

 the damp soil. 



