414 BOTANY OF THE LIVING PLANT 



granular protoplasm. The threads traverse the cell-walls of the 

 host with the greatest ease, while collapse of the cells and loss of 

 mechanical firmness lead to the falling over of the diseased plant 

 (Fig- 349)- 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 are coarse enough to be 



FIG. 349. 



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.) 



seen with the naked eye. The affected seedlings soon become a putrid 

 mass of decay. The fungus that causes the trouble is Pythium 

 debaryanum, which belongs to the large family of the Saprolegnieae. 

 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. 350, c) formed usually from the 



