470 STRUCTURE ANT) 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 (centripetally) , 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 man^r<kinds of food 

 'and fodder plants, and that the organisms causing the 

 tubercles on their roots (pp. 317318) 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 ((Enothera 2 ) , 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." 



2 Called also Onagra. 



