No. 4.] PLANT DISEASES. 33 



and starch ; and of these, the first is water. To grow the 

 })lant, indeed, requires not merely the 80 per cent of water 

 which is stored in the tubers, but vastly more which is given 

 off from the foliage day by day. As nearl}^ as we can calcu- 

 late, there is needed, to produce a crop of 300 bushels of pota- 

 toes per acre, an amount (^f water equal to a layer covering 

 the entire surface of that soil to a depth of 7 inches ; and 400 

 bushels per acre will require an amount equalling a layer 9 

 or 10 inches deep. To supply this requires more water than 

 falls during the season of most rapid groAAi:h. The soil 

 must, therefore, be in such physical condition, owing to 

 humus content and tilth, that it will store the water and 

 yield it upon demand to the roots ; and the husbandman 

 nmst see that there is no loss of this precious water through 

 robbing weeds or wasteful evaporation from the surface of 

 the soil. Hence, clean culture and the earth mulch arc 

 essential to the highest development of the plant, until ])y its 

 own foliage it completely covers the surface of the ground. 

 Lack of sufficient water is the chief cause of sun-scald and 

 tip-burn, — two of the commonest forms of premature death 

 or blighting of potato leaves. Another potent factor con- 

 tributing to this same end is the attack of that minute insect 

 pest the flea beetle, which so riddles many of the leaves 

 about blossoming time that they soon dry out, " burn " and 

 die. Spraying, as described later, will entirely prevent this 

 injury. 



Starch is to be rated next to water in the plant's nutrition. 

 Every grain of starch in the potato tuber was originally made 

 in the green tops, chiefly the leaves. The potato leaf is 

 one of the most marvellous of chemical laboratories, since 

 therein, by a process of which no scientist knoAVs the details, 

 the green tissue, invigorated by the sunlight, absorbs the 

 carbon-containing gas from the air and the water from the 

 roots and combines them — the gas and the liquid — into 

 the solid known as starch. Nature never fails in suppljdng 

 enough air, and rarely is sunlight lacking. The cultivator 

 must see that the water suppl}^ is adequate, as ah*eady indi- 

 cated. The plant naturally develops ample leaves for a 

 bountiful starch production, but the assistance of man is 



