DEVELOPMEiSrT OV TUBERS IX THE POTATO. 



all the tubers in each hill. The minimum size of tuber saved was 

 one-half inch. Where a separation was made into marketable 

 potatoes and culls the division was by weight, the marketable tubers 

 including those equaling or exceeding 3 ounces (above 85 grams), 

 and the culls, those below this limit. 



THE FORMATION OF TUBERS. 



Tuber formation begins, in general, at about the end of the period 

 of flower-bud development, though this is not in all cases an exact 

 criterion. In the earl}^ varieties the tubers invariably start to form 

 by the time the buds are fully developed, while in the late varieties 

 evidences of tuber formation may not be found until the buds have 

 started to open. The 

 Green Mountain va- 

 riety appears to be 

 exceptionally late in 

 this respect, observa- 

 tions having showm 

 that, in Maine at 

 least, the formation 

 of tubers did not 

 start until consider- 

 able bloom was in 

 evidence. 



The first indica-| 

 tions of tuber forma- 

 tion are seen in the 

 swelling of certain lo- 

 calized portions of 

 the stolons. With 

 very few exceptions 

 these enlargements 

 occur only at the ends 

 of the stolons or, if the stolons are branched, at the ends of the 

 branches. At first the swelling assumes a clavate shape but soon 

 becomes globular, and as further development takes place the grow- 

 ing tuber assumes the characteristic shape of the variety. (Figs. 3 to 

 6.) According to Jurgens (6) the form of the tuber is governed by 

 the relation of the length growth to the thickness growth. 



Statistical studies. — To determine the rate of growth of the tubei-s, 

 200 hills of theRuralNew Yorker variety were dug at 1-week intervals in 

 1916 and 1917, begmnmg soon after tuber formation hud started and 

 continuing imtil after the vines had been completely killed by frost. 

 The records of the weight and number of tubers were ol)tained for 

 each hill. In 1917 additional data were obtained from the same 

 material by putting together all the tubers after the weighing of the 



Fig. 3. — Ends of stolons of the Pearl variety of potato, showing 

 different stages in the beginning of tuber formation. Left, 

 stolon before tuber formation has started; center, swelling of the 

 end of the stolon, which is the first stage in the formation of the 

 young tuber; right, a later stage, showing the globular-shaped 

 tuber. 



