PHYCOMYCETES— (a) OOMYCETES 403 



granular protoplasm which contains numerous nucl The threads 

 traverse the cell-walls of the host with the greatest case (Fig. 301), 

 while collapse of the cells and loss of mechanical firmness lead to the 

 falling over of the diseased plant. Left to itself in moist air the disease 

 may spread from plant to plant, the hyphae passing out from the 

 tissues and forming cottony growths through the damp air : they 



Fig. 301. 



Small portion of cellular tissue of a Potato, showing the passage of the hyphae of 

 Pythium through the cell-walls at b. At a, hyphae are seen in an inter-cellular 

 space, one of which has then entered the large cell. Highly magnified. (After 

 Marshall Ward.) 



are coarse enough to be seen with the naked eye. The affected seed- 

 lings soon become a putrid mass of decay. The fungus that causes 

 the trouble is Pythium debaryamim, which belongs to the large family 

 of the Peronosporales. Most of these plants live actually in water, 

 and cause decay in submerged plant- and animal-matter. One of 

 them, Achlya, appears with a high degree of certainty on dead flies, 

 if left floating in foul water. 



Pythium propagates both vegetatively and sexually. The vegeta- 

 tive propagation is by sporangia (Fig. 302, c), formed usually from 

 the ends of the hyphae, by their swelling to an oval form. They con- 



