32 BULLETIN 75, U. S. DEPAETMENT OE AGEICULTUBE. 



cally tripped, of which 35 per cent set jjods, while under natural 

 conditions the same season 13.15 per cent of the flowers developed 

 pods. 



There is a wide range of variability in alfalfa plants as regards 

 the readiness with which the flowers become tripped, either auto- 

 maticaUy or by the aid of external objects, and also in theu* ability 

 to set fruit when tripped. The number of pods set is not propor- 

 tional to the number of flowers, as a smaller proportion of pods is 

 produced on many-flowered racemes than on few-flowered racemes. 



Automatic tripping takes place most frequently in hot sunshine. 

 Humidit}" is doubtless also a factor. Automatic tripping can readily 

 be observed by focusing a burning glass on open flowers or by sim- 

 ply bringmg shaded flowers uito the sunshme on a hot day. 



Insects are the natural agents of cross-pollination in alfaKa, but 

 even where they are scarce, good crops of seed may be produced. 

 Bimiblebees and leaf-cutting bees (Megachile) are the most efficient 

 insects to trip alfalfa flowers. Honeybees secure much honey from 

 alfaKa flowers, but trip only a very small percentage of the blos- 

 soms. Night-flymg insects are of negligible value. Butterflies have 

 never been observed to trip a flower during the course of these studies. 



Rain or wind causes but few alfalfa flowers to become tripped. 



Automatic tripping with consequent self-pollination probably 

 results in the setting of as ' many pods as 'does tripping by insect 

 ^dsitors, at least in the West. This conclusion is also in accord with 

 the observation that excellent seed crops are produced in sections 

 where bumblebees and other insects capable of tripping alfalfa flowers 

 are decidedly scarce. 



O 



WASHINGTON : GOVEUNMENT PRINTING OFFICE : 1914 



