2 BULLETIN 75, U. S. DEPARTMENT OF AGKICULTUKE. 



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 fomid 

 on wliich 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. StiU higher on the stem there were a niunber 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. Xear the tip of the stems nearly all of the 

 flowers fell oft', leaving the stem almost bare of pods. It is probable 

 that this variation m seed production at different periods during 

 the season was due, direct^ 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. 



PREMOUS INVESTIGATIONS OF THE STRUCTURE AND POLLINATION 

 OF THE ALFALFA FLOWER. 



According to Urban, the pecuHar structure of the alfalfa flower 

 by which it trips, or explodes, when visited by certain ulsects was 

 known in the time of Linnseus. The first explanation of the process 

 of explosion is apparently that of De CandoUe,^ 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 aKaHa 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 fertiHzation may also 

 take place in untripped flowei-s, as the pollen may fall on the stigma. 

 His observations were made in Germany. 



In November, 1865, Henslow^ presented a paper before the Lin- 

 nf»'uii Society of London, which, however, was not pubHshed until 

 1867. Hensiow studied carefully the structure of the alfalfa flower 

 with a view of locating the explosive force. This he attributed to 

 the elasticity of the sta-nineal tube, but he was uncertain whether the 



1 Cand j!V , ^ V. de. Physiologio V^g^tale, t. 2, Paris, 1832, p. 548. 



* ' ' ' • T'eher -lie Vorr ichtungen an einigen Bluthen zur Befruchtung durch Inseklenhiilfe. 



B^i :i'b.-Jahjg. 24, No. JO, p. 75, 1866. 



» Uri^, ,j A , ■ -L-orge. Note on the structure of Medicago satlva, as apparently affording facilities for the 

 istercroesinc o! distlnd flowers. Journal, Llnnaean Society, Botany, v. 9, p. 327-329, 1867. 



