" DAMPING-OFF " OF SEEDLINGS 173 



times topple over owing to the dying of the stem 

 just where it emerges from thd soil (Fig. 17, G). This 

 is called " damping off," and is due to the attack of 

 a fungus {Pythium) which has formed an extensive 

 branching mycelium in the tissues of the seedUng. 

 The mycelium itself ramifies in the air spaces (inter- 

 cellular spaces) between the living cells of the host 

 (Fig. 17, A, B), but it pierces the living cells where it 

 touches them (Fig. 17, B), breaking down their proto- 

 plasm. The seedling is soon killed and the fungus 

 lives on for a time in the decaying stem. Eventually 

 the ends of some of the hyphse swell up to form a 

 conidium (Fig. 17, A, sp., and C), which is cut off from 

 the hypha by a cross wall, and becomes detached. 

 In damp air this may grow out at once to form a new 

 mycelium which penetrates a fresh seedling, but in 

 water it puts out a short tube at the end of which 

 a similar body is formed (Fig. 17, D). This is a 

 zoosporangium (cf. Saprolegnia) whose protoplasm 

 divides, to form zoospores (z), which on being set free 

 by the bursting of the sporangium wall swim about 

 in the water. The zoospores germinate {zj) to form 

 new mycelia which penetrate fresh seedlings. Zoospores 

 are never produced except in water. 



Sexual organs are also produced. The female organ 

 is a spherical structure formed by the swelling of the 

 end of a hypha which is cut off from the rest by a 

 cross wall. In this spherical structure the protoplasm 

 contracts to form a smaller sphere (egg). The male 

 organ is club-shaped, and is also cut off from the 

 hypha by a cross wall (Fig. 17, E). 



The male organ apphes itself closely to the female 

 organ {a), and a short tube from the former penetrates 

 the latter (6). Through this the male protoplasm 



