— 
24 BULLETIN 75, U. S. DEPARTMENT OF AGRICULTUR«. 
the hot sunshine automatic tripping took place quickly and could 
easily be observed. These observations remove any possible doubt as 
to the movement being automatic. It will take place, however, only 
when the sun’s heat is intense. It now seems clear that automatic 
tripping is induced mainly by hot sunshine, though it is not proved 
that flowers continually in the sunshine will be tripped to the same 
extent as those alternately in the shade and exposed. 
POLLINATION IN RELATION TO THE RUPTURE OF THE STIGMATIC 
CELLS. 
Burkill’s conclusion that the stigma is not susceptible of pollination 
until the stigmatic cells have been ruptured has already been men- 
tioned. To test this matter further, the following investigations 
were conducted by J. M. Westgate in the greenhouse at Washing- 
ton, D. C., by W. J. Morse at Arlington farm, Virginia, and by 
M. W. Evans at Pullman, Wash. Two methods were used. In the 
first, the standard was removed and the flower then tripped. The 
results of the experiment with this method at the three places 
mentioned are shown in Table XIV. 
TABLE XIV.— The setting of alfalfa pods by flowers tripped after removing the standard. 



| Number of seeds. 
Num- | Number! Number} Flowers 
Observer. Place. Year. | ber of |offlowers| of pods | bearing | 
| plants. | tripped. | formed. pods. | Total. | Average 
| | per pod. 
| Per cent. 
Westgate........- | Washington, D.C.| 1908 18 468 74 ER eee eee oe ieee See Ne 
Morseb2 sas ste Arlington farm....| 1908 5 123 28 22.8 37 1.3 
Doe ace =. 15% aaa Gome 5.0 Se 1909 5) 23 10 43.4 16 1.6 
Evans sacle. el eullman. Wash =! 1909 ses se4 113 14 12545) S52 Le SS eee 


| 


These results show clearly that the mechanical effect of the stigma 
striking the standard is not necessary to insure fertilization. Morse 
also tripped flowers, allowing the stigma to strike the standard. 
In these experiments, 76 flowers in 1908 produced 18 pods, or 23.7 
per cent, and 42 flowers in 1909 produced 22 pods, or 52.4 per cent. 
These figures are but slightly larger than where the standard was 
removed. When the stigma was allowed to strike a piece of wood 
used in tripping, 21 flowers in 1908 produced 3 pods, and 12 flowers 
produced 4 pods—percentages 14.3 and 33.3, respectively. Though 
the numbers of flowers used in the last test were small, the results 
do not indicate that any additional benefit was secured. 
The second method was to remove the standard and then sever 
the keel at the base with arazor. The column thus retained its posi- 
tion in the keel unchanged after tripping. The results of the ex- 
periment with this method are shown in Table XV. 
