DEVELOPMENT OE TUBERS IN 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 early 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 shown 
that, in Maine at 
least, the formation 
of tubers did not 
start until consider- 
able bloom was in A 
evidence. 
The first in die a-" 
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 tubers, 
200 hills of the Rural New Yorker variety were dug at 1-week intervals in 
1916 and 1917, beginning soon after tuber formation had started and 
continuing until after the vines had been completely killed by frost. 
The records of the weight and number of tubers were obtained 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. 
