Feb. 3 ,1933 Use of A Iternating Temperatures in Seed Germination 327 
most complete germination, the rapidity of germination in the chambers 
has been adopted as a basis for comparison with the promptness of 
seedling production in the field. The total percentage of seedling pro¬ 
duction is not considered here, because it is dependent upon other 
factors besides temperature conditions and does not allow of reliable 
comparisons in this connection. 
COMPARISON OP SOII. TEMPERATURES WITH TEMPERATURES IN GERMINATING CHAMBERS 
Figure 20 shows the time-temperature curves of three of the alterna¬ 
tions which are represented in Figure 17, in comparison with those for 
the soil at the depth of 1 inch for four of the days during which the 
field tests were in progress. 
The curves for April 16 and April 29 show the widest and the nar¬ 
rowest daily range in soil temperature occurring during the investigation. 
Both of these days came within a cool period of very slow seedling pro¬ 
duction. The curves for May 18 and June 27 fairly represent the soil 
temperature dturing the 2 periods—the larger part of the month of May 
and the latter part of June—during which the majority of the kinds 
of seed produced seedlings most promptly. 
The soil temperature curves for May 18 and June 27 are similar in 
shape and range to those for chamber temperatures yet show some 
characteristic differences: (1) The lower temperature is well below 
20 0 C. and the range is therefore greater; (2) the rate of cooling is less 
rapid than in case of the alternations which were secured by transfer 
between two chambers (No. 3) and much more rapid after about 5 p. m. 
than in the case of the alternations which were seemed by heating and cool¬ 
ing a single chamber (Nos. 6 and 9), except in the case of No. 4 (fig. 17). 
Both of these soil temperature curves conform to the requirements set 
for temperature alternations in chamber tests of Kentucky bluegrass, 
celery, and tomato seed (seep. 322)—that is, the temperature falls below 
21.5 0 in less than 15 hours after it first rises above 20° and is below 
22.5 0 for more than 12 hours and as low as 20 for several hours of the 
day. Besides, the approximate mean temperatures for the day (23 0 
May 18 and 23.5 0 June 27) are just about the same as for some of the 
more successful alternations, as shown earlier in this paper, and are 
less than for the alternations Nos. 9, 10, and 11, which gave lower 
total germination, and in which the temperature remained relatively high 
during a large part of the day. 
COMPARISON OP TEMPERATURE CONDITIONS GIVING MOST RAPID GERMINATION IN THE 
CHAMBERS WITH THOSE GIVING EARLIEST SEEDLINGS IN THE FIELD 
The smallest number of days required for the production of seedlings 
in the field was 4 days for bean and muskmelon seeds, 5 days for beet, 
pea, squash, and tomato seeds, 6 days for watermelon seeds, 8 days for 
onion seeds, and 12 days for parsley seeds. The average daily range in 
the temperature of the soil at a depth of 1 inch during this period of 
most rapid seedling production was determined by adding separately 
the maximum temperatures and the minimum temperatures for the 
separate days and dividing the sums by the number of days. Table IV 
compares these temperature ranges with those occurring in the chambers 
in case of the temperature alternations giving most rapid germination and 
shows also in each case the approximate number of hours of each day 
during which the temperature remained below 22.5 0 C. 
The average daily range of temperature for most rapid seedling pro¬ 
duction in the soil was in every case greater than for most rapid 
25621—23-3 
