”% BULLETIN 289, U. S. DEPARTMENT OF AGRICULTURE. 
reduction in the quantity of seed required to sow an acre will propor- 
tionately increase the acreage throughout the country which can be 
sown with the available supply of seed. The second line of attack 
has been to determine the environmental conditions necessary to 
the maintenance of a satisfactory stand of clover. That these con- 
ditions are less favorable than they have been in the past is 
indicated by the increasing difficulty experienced by many farmers 
in maintaining clover in the ordinary rotations. A third line of 
attack has been the possibility of developing a heavy-seeding, 
hardy strain of clover with good forage and hay producing qualities. 
The fourth line of attack, the one with which the present publication is 
concerned, has been a study of various means of affecting the yield of 
clover seed under field conditions as they exist throughout the 
clover-growing sections of the country. One phase of this work has 
been to determine the effect of the time of cutting or clippmg the 
first growth on the seed production of the subsequent crop. The 
second phase, that with which this bulletin is primarily concerned, 
has been the effect of various mechanical forms of pollmation upon 
the quantity of seed produced. | 
PREVIOUS INVESTIGATIONS ON THE POLLINATION OF RED CLOVER. 
Since the time of the publication of the statement by Darwin (6, 
p. 361; 7, p. 90)! that 100 heads of red clover on plants protected 
from insects during the blooming period did not produce a single seed 
while a similar number of heads exposed to insects produced an 
average of 27 seeds per head, many scientists have investigated this 
subject. Knuth (22, v. 1, p. 36-37; v. 2, p. 289) accepts Darwin’s 
experimental results and states that red clover, crimson clover, and 
white clover are among the best examples of self-sterility in plants. 
Stebler and Schréter (89, p. 123) in discussing the pollination of red 
clover say that there is no experimental evidence to show that pollen 
from a flower can not, when applied to its own stigma, fertilize the 
ovules, but they also state that pollen which 1s effective in producing 
fertilization has in all probability come from some other flower. The 
same authors (40, p. 14, 122) in a later edition state that red clover 
is self-sterile. Frandsen, according to Lindhard (23), found red 
clover to be practically self-sterile. From 1,235 flowers in 1910 and 
1,305 flowers in 1911, which were self-pollinated by Frandsen, 0.07 
per cent set seed. In 1910 Frandsen pollinated 1,488 flowers and in 
1911, 1,455 flowers with pollen from other heads on the same plants; 
0.8 per cent of the flowers set seed in 1910 and 0.4 per cent in 1911. 
Wallace (43, p. 121) states that insects must perform the indispen- 
sable work of cross-pollinating red clover, but later says (44) that he 
has been inchned to think that climatic conditions rather than the 
1 Reference is made by number to “ Literature cited,” p. 29. 
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