VEGETATIVE AND SEXUAL REPRODUCTION 169 



The stamens are united by the lower part of the filaments, which 

 form a membranous sheath, or stamen tube (d), enveloping the 

 ovary like a sac. Nine stamens are usually thus united, leaving 

 the tenth stamen free. 



The pistil resembles closely the familiar pod of the garden 

 pea and is composed of the inflated ovary, the style, and the 

 stigma. The ovary forms the pod or fruit, and the slender 



Fruit 

 Wing petal Fmieubu 



Ovary 



Keel petals 

 Stigma 



$ 

 f 'Style 



Pistil _ % 



h Cotyledon 



FIG. 89. Structure of the papilionaceous flower of the pea (Pisum) 



a, flower ; 6, irregular petals ; c, stamens and pistil exposed ; d, relation of stamens 

 and stigma ; e, pistil ; /, fruit ; g, seed ; h, embryo and cotyledons 



style bends sharply at its junction with the ovary, thus taking 

 up a position at -right angles to the latter structure. The style 

 is terminated by the somewhat enlarged and roughened stigmas. 

 Mechanism of pollination. The flowers of a large number of 

 species belonging to the pea family are definitely adapted to 

 securing either close-pollination or cross-pOllination through the 

 agency of insects which visit these flowers for nectar or pollen. 

 In some instances, however, the flowers, like those of the garden 

 pea and the sweet pea, are so constructed that self-pollination 

 may habitually occur. In these species the pollen ripens in 



