358 
IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
The time between pollination and fertilization varies with 
the temperature of the atmosphere. The time between pollina- 
tion and fertilization in July is approximately 18 hours, while 
in October it varies from 35 to 50 hours. An examination of 
30 flowers which had been self-pollinated for 55 hours showed 
good germination on the stigmas but no fertilization. The pol- 
len tubes made a slow growth and none exceeded 4 mm. in length. 
In flowers which had been self-pollinated for 90 hours one pol- 
len tube attained a length of 7.5 nun., while the rest were 5 mm. 
or less in length. The pistils of red clover average about 12 mm. 
in length. Eggs were found to be disintegrating four days 
after the flower opened. 
The self-pollination and cross-pollination experiments which 
were conducted in the field checked up very closely with the 
results obtained from the cytological studies. The average yield 
of seed obtained on heads which were not pollinated, and on 
heads which were self-pollinated in different ways, was less 
than one-half of 1 per cent. This small amount of seed may be 
accounted for by the occasional access of bees to these heads for 
a very short time on account of rains or grasshoppers mutilat- 
ing the tarlatan which was used to cover the heads. 
The bumblebee is an efficient cross-pollinator of red clover. 
Bumble bees are able to pollinate from 30 to 35 flowers a minute. 
The honey bee proved to be as efficient a cross-pollinator of 
red clover as the bumble bee in 1911. In 1911 when the pre- 
cipitation was considerably below normal in June, July and 
August and but few nectar producing plants were to be found 
at the time, honey bees collected a large amount of pollen from 
red clover. In order to collect pollen they must spring the keels 
of the flowers. In doing this they cross pollinate the flowers. 
A clover cross-poll enizing machine which was offered for sale 
on the market did not prove to be an efficient cross-pollinator 
of red clover. 
The various types of hand operated brushes which were used 
did not prove efficient as cross-pollinators of red clover. In 
nearly all cases where these brushes were used the seed yield 
was decreased instead of increased. This was undoubtedly due 
to the bristles of the brushes injuring the flowers since the av- 
erage seed yield of the plots given in Table VI II is lower than 
that given in Table IX. The plots given in Table VIII received 
