" DAMPING-OFF " OF SEEDLINGS 173 



times topple over owing to the dying of the stem 

 just where it emerges from the 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 seedling. 

 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 hyphae 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 applies itself closely to the female 

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

 the latter (b). Through this the male protoplasm 



