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the plant in the manner previously described. The 

 living cells are all-important to the plant, as in them 

 the whole of the substance of the plant is manufactured ; 

 and as, in a badly attacked plant, all the matter which 

 would go towards the building up of the tubers is 

 arrested, this would be sufficient to render the crop un- 

 profitable, but as decay is set up also, the plant withers 

 and dies. In mild attacks, when the weather is cool, 



Fig. 19 Longitudinal section of potato-stalk, with germinating zoospore, 

 the germ-tube of which has pierced the cell-wall, and is growing inside 

 the cell. Very highly magnified. Marshall Ward. 



or in varieties which, from their special vigour, are 

 able to combat with the disease, the total destruction 

 of the plant is not effected, and sometimes the disease 

 does not spread to the tubers, but the yield is lessened 

 by the absorption of the newly-formed starch. 



The threads of fungus spread through the plant, and 

 in warm, moist weather, increase with rapidity, and 

 shortly break through the leaf, usually on the under 

 side, because of the greater number of stornata, which 

 form convenient outlets for them. These aerial hyphse 

 develop rapidly, and on these the conidia, or spores, 



