DEVELOPMENT OF PLANTS 439 



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

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

 later period and it is evident that there is a good chance of 

 autogamy as soon as the stigma is mature. Some genera are 

 always autogamous, but it has been observed in many cases that 

 the spores grow more vigorously upon the stigma of another 

 flower than upon the stigma of their own flower, so that if the 

 stigma receives spores from its own and other flowers, the latter 

 spores would develop better and effect fertilization. This is 

 called the prepotency of foreign spores. In some species, au- 

 togamy is impossible owing to the failure of the spores to germi- 

 nate except when transported to other flowers. There was a 

 good illustration of this fact in Australia when the English 

 clover was introduced and flourished exceedingly well, but pro- 

 duced no seed until bees from England were introduced for 

 crossing. Plants transported to foreign countries are often 

 sterile owing to the absence of the insect to which their varia- 

 tions are adapted. There are many variations of the mechanism 

 shown in the pea. In some genera the style and stigma act like 

 a piston, pushing out some of the spores from the apex of the 

 keel with each visit of the insect and in some of these forms the 

 wings are unlocked by the insect when the stamens that have been 

 retained in the keel under considerable pressure are liberated by 

 the dropping of the keel and spring up, scattering the spores with 

 an explosive effect. 



In many of these plants the pod splits at maturity into two 

 valves, each of which twists with a sudden snap in opposite 

 directions, hurling the seeds to a considerable distance (Fig. 

 317, A). This reaction is due to the tension set up by the 

 difference in the drying of the three strata cells which com- 

 pose the walls of the pod. The Papilionaceae contain very many 

 nutritious food plants as the pea, bean and lentils, and also 

 forage plants as the clover, alfalfa and vetches. In temperate 

 regions the majority of the forms are herbaceous, only a few 

 trees and shrubs as the locust (Robinia), broom {Cytissiis), 

 dye weed {Genista), false indigo {Amorpha) belong to the family, 

 but the tropics are richly represented by a great variety of 



