10 BULLETIJSr 958, U. S. DEPARTMENT OF AGRICULTURE. 



These maximum rates were reached dm'mg the week ended August 

 31 m 1916 and September 6 m 1917, or after a growth period, from 

 the time of planting, of 79 and 85 days, respectively^ After these 

 dates the gain per week gradually decreased. Since the first week 

 m September marked the termination! of irrigation in both seasons, 

 it is probable that the falling off in the rate of increase in weight per 

 hill at such a comparatively early stage of the period of tuber devel- 

 opment was due, in part at least, to the diminution of the moisture 

 supply. There was no evidence of maturity of the plants at this 

 time, the Rural New Yorker being a late variety which in this locality 

 remains green and vigorous until killed by frost. In 1917 light frosts 

 on the nights of October 7 and 8 killed the leaves of the plants on the 

 experimental plats and a severe frost on October 4 1 completely killed 

 the vines. The vines were also killed after the next to the last dig- 

 ging in 1916, so that the last increase recorded in each season repre- 

 sents thfi extent of tuber growth that had taken place after the death 

 of the vines. 



An inspection of Table II shows that there was a gradual spread- 

 ing out of the frequency distributions toward the high values as the 

 season advanced until the digging of October 4, after which no 

 important changes occurred. One of the most striking features 

 brought out in this table is the ununif orm rate of growth in the tuber 

 population. The last two samples of the season showed that between 



11 and 12 per cent, or nearly one-eighth of the entir'e number of 

 tubers, had failed to grow beyond the 25-gram class, the average 

 weight of those remaining in this class being 8.2 grams. On the 

 other hand, the data taken at each digging period show a rapid 

 increase in the growth of the tubers of the larger sizes up to the 

 time of the last harvest in September. These inequalities in growth 

 are brought out more clearly in figure 7. It has previously been 

 shown that practically all the tubers which grow to more than one- 

 half inch in size set early in the season, probably at very nearly the 

 same time; therefore, we must conclude that those which are small 

 at the end of the season are of this relative size because of their 

 failure to grow at a normal rate and not because they are of much 

 more recent origin than the larger sizes. 



In Table III, which- gives the weight of the tubers in each of the 

 25-gram classes, the increases in size from week to week are shown 

 by the gradual extension of the distribution and a shifting of the 

 mode toward the higher classes up to October 4, after which there 

 was little change. The failure of part of the small tubers to grow 

 sufficiently to exceed in weight the limits of the 25-gram class, pre- 

 viously referred to, is further emphasized by this table, which shows 

 very little change in the total weight of tubers in this class after 

 September 20. The changes in the distribution with respect to 

 weight resulting from tuber growth are shown in figure 8. 



