DEVELOPMENT OF TUBERS IN THE POTATO. 
25 
The dates of irrigation in both 1917 and 1918 are shown in Table XL 
The number and the weights of tubers have already been shown (Table 
IX) . Considering the averages of all the varieties for each year, it is 
found that in both seasons the early application of water, before tuber 
formation had started, resulted in an increase in the number of tubers 
as well as in the weight per hill. Starting irrigation late in the 
season materially increased the weight over that produced on the 
nonirrigated plats. On the other hand, very little difference is shown 
in the number of tubers per hill produced under the two conditions. 
In 1917 a slight increase was indicated, but the small number of indi- 
viduals involved raises the question whether so small a difference may 
not be due to so-called experimental error. Where irrigation was 
started before the tubers had begun to form, increasing the number 
of irrigations did not increase the number of tubers. In 1918 one 
extra irrigation increased the weight of tubers 33 grams per hill, 
while in 1917 four extra irrigations resulted in a decrease in weight. 
Apparently the quantity of water applied in this case was in excess of 
the optimum requirements of the plants. The results in general, 
while not of sufficient extent to warrant final conclusions, indicate 
that each application of water, at almost any period in the growth 
of the plant, provided excessive quantities are not used, may be ex- 
pected to produce an increase in the weight of the crop, but that little 
or no increase in the number of tubers is likely to result from irrigation 
after tuber formation is well started. 
Table XI. — Dates of irrigation in 1917 and 1918 as related to the production of potato 
tubers of different varieties grown at Greeley, Colo., in 1917 and 1918. 
Treatment. 
Experiments in 1917. 
Experiments in 1918. 
June. 
July. (August. 
Sep- 
tember. 
July. 
August. 
Sep- 
tember. 
First irrigation after tubers had begun to set. . 
14, 19, 
and 31. 
...do... 
...do... 
7 
7 
7 
6 
6 
21 
8, 15, 
and 30. 
8, 15, 
25, and 
30. 
4 
27 
3, 15, 
21, and 
27. 
4 
Irrigation begun early, many irrigations 
21 
4 
SUMMARY. 
The time of the beginning of tuber formation was found to coincide 
very closely with the end of the period of flower-bud development. 
The statistical studies of tubers at 1-week intervals showed that 
the greater part of those which grew to exceed half an inch in diam- 
eter were formed at the beginning of the period of tuber development 
The maximum rate of growth of the tubers was found to occur 
about the last of August or first of September, which was approxi- 
