34 DISEASES OF TROPICAL PLANTS oa. 



on which are borne the sporangia containing great 

 quantities of non-sexual spores. When these upright 

 branches or aerial hyphae are forming, they are white, 

 and the bread has the appearance of being covered with 

 loose cotton. As the spores mature, the sporangia turn 

 black and eventually the whole mass falls and blackens. 

 When it becomes dry the spores are so light that they 

 float readily in the air and are carried about until they 

 fall under suitable conditions, when they grow and 

 produce new plants. The sexual reproduction is com- 

 paratively uncommon ; two short hyphal branches, 

 which so far as we can determine are practically alike, 

 are produced. These branches grow until they unite 

 and then form a thick-walled spore, known as a zygo- 

 spore or resting-spore. These sexual spores are rather 

 uncommon in many species but they are no doubt 

 capable of carrying the organism over very long periods 

 of time which are unfavourable for growth. 



This species is the cause of a very severe rot on both 

 the white and the sweet potatoes (page 238). Acrocystis 

 batatas, E. & H., is another species belonging to the 

 Mucorales, and is the cause of another rot of the sweet 

 potato. Other species of the mucors are more or less 

 common on dungs, ripe fruits, and stored vegetables, and 

 are frequently the causes of considerable losses. 



Order 3. ENTOMOPHTHORALES. Most species of this 

 order are parasitic on insects, which they sometimes 

 destroy in great numbers. They no doubt hold insects 

 in check in some measure, and are therefore beneficial. 

 The body of the insect becomes filled with mycelium, 

 and after its death the fungus continues to grow sapro- 

 phytically, breaks through the body wall, and produces 

 a number of sporophores, each of which bears a single 

 conidium/ The sexual spores are occasionally formed 

 in a manner similar to the mucors. 



Under Sub-class 3, the Oomycetes, will be considered 

 the orders Saprolegniales and Peronosporales. 



Order 4. SAPROLEGNIALES. The majority of species 

 in this order are aquatic parasites or saprophytes and 



