26 
IOWA ACADEMY OP SCIENCE 
The results of these investigations are shown in the accompanying tables. 
The general effect of exposing the seeds to thawing and freezing was both to 
increase the percentage of germination and to shorten the dormant period. 
This was especially true of seeds with hard seed coats. Where the seed pos- 
sessed thin and delicate seed coats the effect in a few cases was to lessen or 
destroy the vitality of the seeds. 
Three samples of the common pigweed (Amarantus retroflexus) showed 
an average dormant period of nine and one-third days for those kept indoors 
and only six and one-third days for those exposed, an increase of about 50 
per cent. For Wild Rye (Elymus canadensis) the dormant period was les- 
sened from nine to five days and the percentage increased from 22 to 48 on ac- 
count of exposure. In four samples of Foxtail {Setaria glauca) the average 
dormant period was lessened from eleven and one-quarter to seven days and 
the percentage of germination increased from 34.5 to 38 per cent because of ex- 
posure. 
It is to be seen from an examination of the tables of November and De- 
cember that as a rule the highest dormant periods are found in those seeds 
that have the hardest and thickest seed coats. For these two months, Rag 
Weed (Ambrosia trifida) and Barn Yard Grass (Panicum crus-galU) h'ave the 
longest dormant periods, that for the Rag Weed being one hundred and fifty- 
two days and that for Barn Yard Grass one hundred and seventy-eight days. 
It was late in the spring before either would germinate. It is seen also from 
the tables that for some of the hard seeded species the dormant period de- 
creases in each succeeding month not quite in proportion to the time between 
the successive plantings. This shows that these species refuse to germinate 
even under the most favorable conditions until they have had a period of rest. 
The tables also show a general falling off of the percentage of germination for 
those seeds planted in January and February, as compared with the two months 
preceding and the two months following. This indicates that there are two 
natural periods for the best seed germination, the fall and the spring. 
The entire test experiment shows a very low average per cent of germina- 
tion, although care was taken to collect seeds from healthy, mature plants. This 
suggests one of two things, either that the average per cent of fertile weed 
seeds is low or that many seeds quickly lose their vitality if they are not allowed 
to come in contact with, moist soil. 
The highest average percentage of germination, as well as the shortest dor- 
mant periods, is shown by the common mustard {Brassica sinapistrum) . The 
percentage of germination for November and December was 100 per cent and 
for the six months 90.3 per cent. Both Mustard and Yarrow showed a gradual 
decrease in vitality from month to month, while the dandelion showed a 
gradual increase. 
The general conclusion to be drawn from these experiments is that most 
weed seeds with thick seed coats require a more or less extended period of 
rest after maturity, that the seed of the Mustard and Pepper Grass require 
little time for rest, that the vitality of nearly all weed seeds is weakened by 
drying out and that the power of germination is increased by exposure to the 
natural periods for the best seed germination, the fall and the spring. 
