DEVELOPMENT OF PLANTS 



453 



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

 characteristics of this form of flower are very well illustrated 

 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 



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. 



ovary which terminates in a stylar brush of upwardly pointing 

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

 the tip of the keel (Fig. 316, B, D). The significance of these 

 features will be discovered if you watch a bee probing into the 

 sheath of filaments after the nectar secreted at the base of the 

 ovary. The standard serves to attract and also direct his ap- 

 proach to the flower so that he alights at the proper place, i. e.^ 

 on the keel and wings. The weight of his body causes a slight 

 depression of the keel and the stylar brush sweeps some of the 

 microspores upon his body. It should be added that the flower 

 is ready for the insect as soon as opened since the spores are 

 discharged before the flower blooms. Only a portion of the 

 spores are swept out by the insect and the dusting may be re- 

 peated on several visitors. The stigma does not mature until a 



