330 
Journal of Agricultural Research voi. xxm. No. 5 
SUMMARY 
(1) Seeds of carrot, parsley, timothy, awnless brome grass, perennial 
and Italian rye grasses, meadow fescue, and several kinds of flower seeds 
germinate practically as well at favorable constant temperatures as with 
an alternation of temperatures. 
(2) Seeds of redtop and parsnip and sometimes seeds of petunia 
germinate somewhat better, and seeds of celery, orchard grass, Kentucky 
bluegrass, Bermuda grass, and Johnson grass germinate much better 
with favorable alternations of temperatures than at constant tempera¬ 
tures. The exact alternation giving best results depends upon the kind 
of seed and to some extent also upon its physiological condition. 
(3) Different lots of the same kind of seed sometimes vary widely in 
temperature sensitiveness. It may therefore be that some kinds which 
are usually constant-temperature germinators may, under certain con¬ 
ditions, germinate better with an alternation of temperatures. Incom¬ 
plete after-ripening might have this effect. 
(4) Several hypotheses have been suggested which may help to ex¬ 
plain the effect of alternating temperatures upon germination, but none 
of them can be considered adequate without more definite evidence than 
is now available. 
(5) The favorable effect of an alternation of temperatures upon the 
germination of certain kinds of seed can not, with our present knowledge, 
be referred to the specific effect of the extreme temperatures of the 
alternation or of the mean temperature of the alternation but are the 
result of the changes in temperature. In the case of Kentucky blue- 
grass and Johnson grass seed, at least in some cases, the effect of the mean 
temperature of the alternation supplements the effect of the alternation 
as such. 
(6) On account of greater ease, simplicity, and uniformity of tempera¬ 
ture control, coupled with equally good germination results, the method 
of securing temperature alternations by transfer between two germinat¬ 
ing chambers at fixed temperatures is preferable to the method of heating 
and cooling a single chamber. 
(7) In title use of temperature alternations the upper temperature 
should be maintained only a small part of the day, never more than 
eight hours and usually not more than six hours, and the change to the 
lower temperature should then be fairly rapid. These results are easily 
attainable by the transfer method. 
(8) An alternation between 20° C. for 16 to 18 hours and 30° for 6 to 8 
hours each day gives good results in the germination of parsnip, celery, 
redtop, and orchard-grass seed and seems to be an optimum temperature 
condition for germination of Kentucky bluegrass seed. 
(9) A large percentage of at least some lots of celery seed will germinate 
at nearly constant low temperatures (15 0 to 20° C.), but germination is 
then so slow as to make the use of a warmer alternation preferable. 
(10) A similar alternation between 20° and 35 0 C. gives good results 
with Bermuda grass seed; an alternation in a still warmer temperature 
range might possibly be still better. 
(11) For the germination of Johnson grass seed the very warm alter¬ 
nation 30° C. for from 18 to 22 hours and 45 0 for from 2 to 6 hours each 
day is best. 
(12) Those lots of Johnson grass seed which germinate most readily 
under favorable temperature conditions are less sensitive to temperature— 
that is, they germinate well under a wider range of temperature con- 
