PLANT DISEASES 281 



life history, but they have this feature in common, that 

 they pass most of their life in the soil and usually are 

 able to live saprophytically, that is, on the dead material 

 of certain other plants. They also all possess the power 

 of becoming parasites by attacking the seedling plants. 

 Another feature which these fungi have in common is 

 that they thrive best under moist conditions. Low 

 situations, therefore, water-logged soil, poor drainage, 

 or bad ventilation, as in a greenhouse, will all con- 

 tribute to the spread of this disease. 



The life story of one of the most common and best 

 known of these fungi may be given as an illustra- 

 tion of how they live. This fungus is commonly known 

 as damping-off fungus (Pythium debaryanum). It 

 produces comparatively large threads, which grow in the 

 soil and under proper conditions give rise to little cases 

 which finally break and let out tiny spores that are 

 provided with whiplike lashes. These lashes enable 

 the spores to swim around in the water. The spread 

 of this fungus is therefore made easy by an abundance 

 of water in the soil or at the surface of the ground. 

 These swimming spores finally come to rest, pull in 

 their lashes and grow out into threads which lengthen 

 and branch in the usual way and finally build up a mass 

 of threads, that is, a mycelium. If the soil dries up 

 or other conditions come about which are unfavorable to 

 the growth of the fungus, the threads may form another 

 kind of spore which may be called a resting spore, or 

 winter spore. This spore is quite large and has a very 

 thick wall, so that its contents are well protected. It 

 can endure a great deal of drying out and cold, and will, 

 when favorable conditions return, again break up into 

 swimming spores. These swimming spores grow as 



