peb. 3.19*3 Use of AIternating Temperatures in Seed Germination 307 
As germination advanced the alternations proved more advantageous 
and the shape of the graphs representing germination became reversed, 
with the low points falling on the lines for constant temperatures. 
The alternation 30° to 45 0 C. gave best results, and 25 0 to 40° next 
best. 5 With the alternation 30° to 45 0 germination was more rapid at 
first, progressed at a more nearly uniform rate, and was more active in 
the last 10 days of the germination test than with the alternation 25 0 to 
40°. With the latter alternation nearly all of the germination occurred 
between the second and the sixth days. 
As with Kentucky bluegrass seed (see p. 301), the effect of the mean 
temperature of the alternations supplements in a definite manner the 
more important effect of the temperature changes as such. The alter¬ 
nation 20 0 to 30° C. evidently is not wide enough to produce the desired 
results. Table II compares the other alternations used and the percent¬ 
ages of germination on the basis of the approximate mean temperatures 
of the alternations. The percentage of germination rose regularly with 
rising mean temperature up to 34 0 and lien fell abruptly. 
At 40° C. constant, germination was spread over an even longer period 
than with the alternation 30° to 45 °. It is important to remark here that 
while some seeds produced normal seedlings at this high temperature 
(40°) many at this temperature and a few at a constant temperature of 
35 0 germinated only sufficiently to spread the heavy scales which inclose 
the caryopses by the slight elongation of radicle or coleoptile, after which 
growth ceased. These seeds were counted as germinated. When such 
seeds, after growth had stopped, were placed at a cooler temperature, 
either constantly or in alternations, normal growth ensued. One is re¬ 
minded here of Kidd & West's work (14) in which a variety of agents 
induced growth in dormant white mustard seeds when used in concen¬ 
trations or degrees just falling short of causing serious injury to the seed. 
According to this conception heat might here be considered as a stimulus 
to germination; and the advantage of an alternation between tempera¬ 
tures, the warmer of which lies either near the upper limit of endurance of 
a given seed or above the optimum for its germination and the subsequent 
growth of the seedling, might lie in the fact that the seed is given the tem¬ 
porary and recurring advantage of an elevation of temperature without 
being subjected to the harmful effects of long-continued exposure to a 
high temperature. In this connection it should be remembered also that 
in the case of a temperature alternation the actual temperature of the 
seeds probably never quite reached the temperature shown on the ther¬ 
mometer in the warmer germinating chamber and that the rise and fall 
in the temperatures of the seeds were relatively slow and gradual. 
Table II .—Temperature alternations, corresponding mean temperatures, and average 
percentages of germination produced in six lots of Johnson grass seed 
Temperature alternation. 
Approxi¬ 
mate mean 
tempera¬ 
ture. 
Average 
percentage 
of germi¬ 
nation. 
Temperature alternation. 
Approxi¬ 
mate mean 
tempera¬ 
ture. 
Average 
percentage 
of germi¬ 
nation. 
°c. 
15 t0 3°. 
20 to 35 . 
20 to 40. 
°c. 
19 
24 
25 
41 
Si 
63 
°c. 
25 to 40. 
30 to 45. 
35 to so. 
°c. 
29 
34 
39 
78 
88 
36 
* Auxiliary viability tests of the seeds remaining ungerminated with the alternation 30° to 45 0 C. were 
made by removing the scales and pricking according to a method previously described (it) . They showed 
for the seeds originally put to germinate an average viability of 93 per cent, which is only 4 per cent 
greater than the germination without special treatment. 
