2 BULLETIN 75, U. S. DEPARTMENT OF AGRICULTURE. 
Observations at the same place also indicate that the factors or 
conditions which favor seed production vary during the season, as 
shown by the distribution of pods on mature plants. For instance, 
in the latter part of August, 1910, a great many plants could be found 
on which the earliest racemes to develop in the spring, located at the 
base of the plant, produced large numbers of pods. A little higher 
on the plant most of the flower stalks were almost or entirely bare 
of pods. Still higher on the stem there were a number of large 
well-filled clusters of pods, indicating that for a period of two weeks 
or more preceding August 20 a very large proportion of the flowers 
had developed pods. Near the tip of the stems nearly all of the 
flowers fell off, leaving the stem almost bare of pods. It is probable 
that this variation in seed production at different periods during 
the season was due, directly or indirectly, to climatic conditions. 
At Arlington farm, Virginia, it has frequently been observed that a 
large proportion of the pods fail to set, even when the flowers have 
been artificially tripped. While this is especially true of the flowers 
of the first crop of alfalfa, it seems to be due more to adverse climatic 
conditions than to the vigor of the plants. 
PREVIOUS INVESTIGATIONS OF THE STRUCTURE AND POLLINATION 
OF THE ALFALFA FLOWER. 
According to Urban, the peculiar structure of the alfalfa flower 
by which it trips, or explodes, when visited by certain insects was 
known in the time of Linneus. The first explanation of the process 
of explosion is apparently that of De Candolle,’ in 1832. De Candolle 
states that the explosion of the flower takes place when a certain 
stage of its maturity is reached. ; 
Hildebrand,” in 1866, gives a brief general account of the structure 
of the alfalfa flower, comparing it with both Indigofera and Cytisus. 
He clearly recognizes that the peculiar mechanism of the flower is 
an adaptation for pollination by insects, but states that inclosed 
flowers finally trip in the course of their development without the 
help of insects. Apparently he considers that fertilization may also 
take place in untripped flowers, as the pollen may fall on the stigma. 
His observations were made in Germany. 
In November, 1865, Henslow? presented a paper before the Lin- 
nean Society of London, which, however, was not published until 
1867. Henslow studied carefully the structure of the alfalfa flower 
with a view of locating the explosive force. This he attributed to 
the elasticity of the stamineal tube, but he was uncertain whether the 
1Candolle, A. P.de. Physiologie Végétale, t. 2, Paris, 1832, p. 548. 
2 Hildebrand, F. Ueber die Vorrichtungen an einigen Bliithen zur Befruchtung durch Insektenhiilfe. 
Botanische Zeitung, Jahrg. 24, No. 10, p. 75, 1866. 
3 Henslow, George. Note on the structure of Medicago sativa, as apparently affording facilities for the 
intercrossing of distinct flowers. Journal, Linnean Society, Botany, v. 9, p. 327-329, 1867. 
