SWEET-CLOVER SEED. 3 
in 1904 died, Kirchner concluded that the flowers of M. officinalis 
were especially sensitive to inclosure in nets and that the f ailures to 
obtain more than a very few seeds on protected racemes in Darwin's 
experiment and in his first experiment were due to this cause. 
According to Kerner (17, v. 2, p. 399) the peas and lentils (Pisum 
and Ervum) and the different species of horned clover and stone 
clover (Lotus and Melilotus) as well as the numerous species of the 
genus Trifolium and also many others produce seeds when insects 
are excluded from the plants, and only isolated species of these 
genera gave poor yields without insect visitation. 
OUTLINE OF POLLINATING EXPERIMENTS. 
The yield of sweet-clover seed varies greatly from year to year in 
many parts of the United States. It has been assumed that this 
variation was due to climatic conditions, as excellent seed crops were 
seldom harvested in seasons of excessive rainfall or of prolonged 
drought just preceding or during the flowering period. The lack of 
a sufficient number of suitable pollinating insects also was thought 
to be an important factor in reducing seed production. This was 
especially true where the acreage of sweet clover was large and where 
few, if any, honeybees were kept. 
In order to obtain data upon the factors influencing the yield of 
seed, a series of experiments was outlined to determine (1) whether 
the flowers are able to set seed without the assistance of outside agen- 
cies, (2) whether cross-pollination is necessary, (3) the different kinds 
of insects which are active agents in pollinating sweet clover, and (4) 
whether a relation exists between the quantity of moisture in the soil 
and the production of seed. 
The racemes containing the flowers which were to be pollinated by 
hand were covered with tarlatan before any of the flowers opened and 
were kept covered except while being pollinated until the seeds were 
nearly mature. This cloth has about twice as many meshes to the 
linear inch as ordinary mosquito netting and served to exclude all 
insects that are able to pollinate the flowers. When entire plants 
were to be protected from all outside agencies, cages covered with 
cheesecloth, glass frames, or wire netting were used. 
A preliminary study of the pollination of Melilotus alba and M. 
officinalis showed that both were visited by the same kinds of insects 
and that both required the same methods of pollination in order to 
set seed. On this account M. alba was used in most of the experi- 
ments reported in this bulletin. Where M. officinalis was employed 
it is so stated. 
