24 DISEASES OF TROPICAL PLANTS 



CH. 



which two individuals or parts of individuals, or two 

 parts of the same plant (gametes) unite. 



The non-sexual method is by cutting off of a small 

 fragment of the mycelium ; but this may be accomplished 

 in many ways. Some of the more common methods 

 are : 



(a) The formation of swarm spores within a part 

 of the mycelium. By this method a cell wall is formed 

 separating a part of the mycelium into a separate cell. 

 Then the contents divide into a large number of small 

 cells, each of which is provided with small cilia or thread- 

 like processes, which enable it to swim in the water 

 or in a very thin film of moisture on the surface of a 

 plant. After swimming for a time they attach them- 

 selves and grow, eventually becoming mature fungi 

 like the parent by which they were 



produced. Such spores are sometimes 

 called zoospores because of their resem- 

 blance to little animals (Fig. 13, Sapro- 

 legnia). 



(b) The formation of conidia, either 

 solitary on the ends of simple branched 



FIG. 13. The sporangia of Saprolegnia, 

 showing the formation of zoospores. 



FIG. 14. Conidiophores 

 and conidiospores of 

 downy mildew. 



hyphae (Fig. 14, Downy Mildew), or in chains (Fig. 15, 

 Powdery Mildew ; Green Mould). These spores may 

 produce swarm spores by internal cell division as above, 

 or they may grow directly into new plants. 



