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A TEXT-BOOK OP BOTANY 



ing upon a suitable substratum germinate and produce new 

 mycelia. These spores, although asexual, are evidently not 

 swimming spores, as there is no water medium for them to 

 use. This method of transfer being impossible, the spores 

 are scattered by currents of air, and must be correspond- 

 ingly light and powdery. It is interesting to note that cer- 

 tain molds that grow in the water develop swimming spores. 

 While the ordinary method of reproduction through the 

 growing season is by means of these rapidly germinating 

 spores, in certain conditions sexual reproduction also occurs. 

 Branches put forth from two contiguous mycelial threads, 

 the tips of the branches being in contact (Fig. 130, A). 

 Partition walls separate the tips from the main body of the 

 plant (Fig. 130, B), the walls in contact become perforated, 

 the contents, of the two tips fuse, and a heavy- walled 

 oospore is the result (Fig. 130, C).* This sexual process 

 suggests that of Spirogyra ( 69). 



80. Peronospora. These are the downy mildews, very 

 common parasites on the leaves of seed-plants. The 

 _ mycelium is entire- 



ly internal, branch- 

 ing among the tis- 

 sues of the leaf, and 

 piercing the living 

 cells with sucker-like 

 branches that rap- 

 idly absorb their 

 contents (Fig. 131). 

 The presence of the 

 parasite is made 



known by discolored and finally deadened spots on the 

 leaves, where the tissues have been killed. 



* It is not easy to induce Mucor to perform the sexual process, and 

 in fact such a process may not often occur in nature. 



FlG. 131. Branch of mycelium of Peronospora in 

 contact with two cells of a host plant, and send- 

 ing into them absorbing branches. After DE 

 BARY. 



