DEVELOPMENT OF TUBERS IN^ THE POTATO. 



25 



The dates of irrigation in both 1917 and 1918 are shown in Table XI. 

 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. 



Tablk XI. — Dates of irrigation in 1917 and 1918 as related to the production of potato 

 tubers of different varieties groini at Greeley, Colo., in 1917 and 191S. 



Treatment. 



First irrigation after tubers had begun to set. 



Irrigation l)egun early, niediuni number 



Irrigation l)egini early, many irrigations 



Experiments In 1917. 



June. 



July. August 



27 



3, 15, 



21, and 



27. 



14, 19, 

 and 31. 



...do... 

 ...do... 



Sep- 

 tember. 



Experiments in 1918. 



Julv. August 



6 I 8, 15, 

 and 30. 



8, 15, 

 I 25, and 

 3a 



Sep 

 tember. 



SUMMARY. 



The time of the beginning of tuber formation was found to coincide 

 very closely with the end of the period of flower-bud developinent. 



The statistical studies of tiil)ors at 1-week intervals showed that 

 the greater part of those which grow tt) 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 Jirst of September, which was approxi- 



