420 



THE ROSALES 



very easy transition from the regular flower of the plum to the 

 highly modified flowers of the pea family. 



(b) The Pea Family, Papilionaceae. The pea family (Fig. 

 316) is the highest of the rose order and the largest family, with 

 one exception, of all the angiosperms, comprising over 11,000 

 species. Here we find the same type of flower as in the redbud. 

 The standard in this family, however, incloses the wings and 

 the stamens, usually ten in number, may be distinct as noted 

 above or united by their filaments into a sheath about the soli- 

 tary pistil, more frequently a single stamen remains free, an 

 arrangement called diadelphous (two brotherhoods). This type 

 of flower is called papilionaceous from its fancied resemblance 

 to a certain genus of butterfly, Papilio. The more important 

 characteristics of this form of flower are very well illustrated 



FIG. 316. Structure of the sweet pea (Lathyrus) : A, flower of the pea 

 c, calyx; s, standard enclosing the two wings, w; k, keel. B, section of 

 flower, showing the sporophylls concealed in the keel. C, one of the wings. 

 D, perianth removed, showing relation of sporophylls. 



in the pea (Fig. 316). The standard is the conspicuous and 

 most highly-colored organ of the flower, overlapping the two 

 wings which in turn practically cover the keel. By carefully 

 removing the keel and wings, it will be seen that these organs 

 are attached to the calyx and receptacle by rather narrow claws 

 (Fig. 316, C) and that they are also locked together by a little 

 process on each wing that fits into a groove on the keel. Nine 

 of the filaments, the tenth being free, form a sheath about the 

 ovary which terminates in a stylar brush of upwardly pointing 

 hairs and in a stigma. The anthers and style are confined in 



