470 STRUCTURE AND LIFE HISTORIES 
visit would not be profitable. Other students regard this 
as questionable. 
Since, as shown by examining a bud, the flowers of the 
clover head mature from the circumference toward the 
center (centripe tally) , the outer flowers are the first to be 
visited, and hence the first to bend down (Fig. 353). 1 
Many of the papilionaceous legumes are self-pollinated. 
This is true of the "sweet pea" as a rule, but not without 
exceptions. 
The fact that the legumes furnish so many kinds of food 
and fodder plants, and that the organisms causing the 
tubercles on their roots (pp. 317-318) are an important 
source of the nitrogen necessary for successful agriculture, 
renders this family one of the most important of all the 
economic plants, possibly exceeded only by the grass family. 
415. Evening-primrose Family (Onagraceae). The 
evening-primroses have recently come into very great 
prominence on account of the fact that they have been 
extensively used for the experimental study of evo- 
lution. A knowledge of their structure has, therefore be- 
come increasingly important. The flowers are perfect 
and symmetrical, with the parts usually in fours. The 
ovary is one- to six- (usually four-) chambered, and the 
calyx tube adheres to the walls. The stamens are in- 
serted commonly on the summit of the calyx tube. In 
the evening-primrose itself (QLnothera?} , the pollen- 
grains are held together by delicate threads that resemble 
a cobweb. The seedling usually forms a rosette the first 
year, and thus passes the winter (Fig. 354). The follow- 
1 Often one will find a solitary, unpollinated flower left standing, and in 
some localities these are sought by children as "old-maid clovers." 
1 Called also Onagra. 
