Alfalfa Seed Yields 
27 
There is a very wide range in type of plants grown from common commercial seed. The 
plants shown are typical variations obtained from the planting of common commercial seed. 
‘Uhrips” has been regarded by some as an important factor in caus- 
ing alfalfa seed failures. Careful examination of flowers on plants 
that were setting seed successfully showed that the thrips were as 
abundant on such flowers as on blooms where no seed or little seed 
was setting. 
From these investigations, it seems probable that some of the 
iiijury to alfalfa bloom is due to the thrips. There is some injury 
from other insects. But it is hardly possible that these injuries are 
the prime cause of seed failure. This conclusion is reached because 
many of the worst injured blooms set fairly well with seed. 
In seasons of severe alfalfa see4 failure, it is usually possible to 
find a few plants which set seed perfectly, even where the surround- 
ing plants in the field are a dismal failure. Such results in the 
midst of failure is the paradox in the alfalfa seed problem. It is 
difficult to harmonize these 'results with any theory of insect activities 
in relation to alfalfa seed setting. If beneficial insects are essential 
to fertilization of the alfalfa flowers, it would not seem possible that, 
a few plants would be well fertilized, while the mass of adjacent 
plants were not. On the other hand, if injuries were responsible for 
seed failures, it would seem incredible that some plants would escape 
injury entirely, that their flowers should become well fertilized. 
