BOOT CROPS. 73 



affected soon rot, and have a foetid odour, so that they are 

 not only useless themselves, but communicate the disease 

 more or less to the whole crop. The cause of " clubbing " 

 was proved by M. Woronin in 1876 to be due to a slime 

 fungus belonging to the Myxomycetes. " The Myxomy- 

 cetes are especially remarkable, from the fact that they do 

 not form cells, cell-walls, ' tissues ' or mycelium, during 

 the period of vegetation; but their protoplasm remains 

 during that time free, and collected into small masses 

 of various and changeable forms. At a certain definite 



■" 2. 



%\4 



Fig. 31. — " FiNGEB-AND-ToE " OF TuENips {Plasvwdiophora braBsiccs). 



1. Transverse section of turnip root, showing the " plasma" (B) of the fungus. 



A. Normal size of cell. B. Cell distended by fungus (x 100). 



2. Spores, produced from plasma ( x 195). 



3. Spores giving rise to zoospores (a) (x 490). 



advanced period of growth the vital material of a Myxo- 

 mycete breaks up into small portions, and these portions 

 at length surround themselves with a cell-wall, and 

 become either fruits, sporangia, or spores ; and in this 

 condition the fungus remains at rest during a certain 

 definite period." This is essentially different from the 

 life-history of Peronospora infestans or any other fungus 

 previously described. 



