THE LATE OR MAIN-CROP VARIETIES 467 



tubers ; but tubers may be cut into small pieces, each of which 

 may be planted provided that it bears one or more ' eyes,' from 

 which stems may arise. 



From 12 to 18 cwt. of 'sets' are needed to plant an acre. 

 Early varieties are planted in February and March, later ones 

 in April, in drills from 24 to 27 inches apart, the tubers being 

 placed about 15 inches asunder in the rows. 



As far as possible the drills should run north and south, on 

 somewhat stiff soils inclined to dampness; on drier soils east 

 to west. 



Yield. — The average yield per acre is 7 or 8 tons. 



Composition. — The most important ingredient in the tuber 

 is starch, the amount of which varies from 10 to 26 per cent. ; 

 the best varieties usually contain about 18 to 22 per cent. 



Sugar is absent from well-ripened tubers, and there is only a 

 trace of fat in them. 



The nitrogenous substances average a little over 2 per cent., 

 of which about i'2 are albuminoids, present in the protoplasm, 

 in solution in the cell-sap, and also in the form of solid ' proteid- 

 crystals.' The latter occur chiefly in the cells of the cortex. 



The water-content averages about 75 per cent. 



A poisonous substance solanin is present in nearly all parts of 

 the plant, the young etiolated shoots of the tuber and the berries 

 containing most. 



Ex. 238. — Sow true seeds of potato plant in boxes or pots of earth, and ex- 

 amine at different stages of growth. Note the form of the cotyledons, the 

 extent and position of the root, and the origin of branches which bear tubers. 



Ex. 239.— Examine the arrangement of the ' eyes ' on a large, long, 

 coarse tuber, and note the relative number at the ' heel ' and ' rose ' 

 end respectively. 



Cut longitudinal and transverse sections of the tuber, so as to pass through 

 one of its ' eyes,' and note the cortex, vascular part, and irregular outline of 

 the medulla. 



Ex. 240. — -Examine several sprouted tubers which have been allowed to ger- 

 minate in the dark on a stable or cellar floor without touching each other. 



