32 BULLETIN 75, U. S. DEPARTMENT OF AGRICULTURE. 
cally tripped, of which 35 per cent set pods, 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- 
matically or by the aid of external objects, and also in their 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. 
Humidity is doubtless also a factor. Automatic tripping can readily 
be observed by focusing a burning glass on open flowers or by sim- 
ply bringing shaded flowers into the sunshine on a hot day. 
Insects are the natural agents of cross-pollination in alfalfa, but 
even where they are scarce, good crops of seed may be produced. 
Bumblebees and leaf-cutting bees (Megachile) are the most efficient 
insects to trip alfalfa flowers. Honeybees secure much honey from 
alfalfa, flowers, but trip only a very small percentage of the blos- 
soms. Night-flying 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-pollmation probably 
results in the setting of as many pods as does tripping by insect 
visitors, 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 : GOVERNMENT PRINTING OFFICE: 1914 
