THE FLOWER. 



to jostle some of the pollen down upon its own stigma, so that 



self-fertilization is not uncommon. 



418. Apios tuberosa, a near relative of Phaseolus, exhibits a 



different and equally curious modification of the same parts. 

 The wing-petals for landing place 

 are similar : the standard is pro- 

 portionally large, firm in texture, 

 and shell-shaped or concave, with 

 a small boss at the tip as seen from 

 behind, or a shallow sac as seen 

 450 ^ from the front : the keel is narrow 



and sickle-shaped ; it arches across the front of the flower, and 



the blunt apex rests in the notch or shallow sac of the tip of the 



standard. (Fig. 450, 452, 453.) So it remains if untouched until 

 the blossom withers : no self-fertilization has ever been observed, 

 and none ordinarily occurs. The anthers 

 are assembled close around the stigma, 

 but a little short of it (Fig. 452) ; the 

 pollen is not early nor copiously shed in 

 the enclosure : the small terminal stigma 

 is at first covered with a pulpy secretion, 

 which at length collects into a soft ring 

 around its base over or through which no 

 pollen passes. But when the keel is liber- 

 ated by lifting from underneath, it curves 

 promptly into the shape shown in Fig. 



FIG. 450. Flower of Apios tuberosa, unvisited. 451. Same after visitation, the 

 keel dislodged from the retaining notch, and more incurved; the tip of the style pro- 

 truded and thrust forward, followed by the anthers. 



FIG. 452. Enlarged vertical section of flower-bud of Apios tuberosa. 453. A flower 

 with half the standard cut away, to show the blunt apex of the keel resting in the 

 notcfc. 454. Diagram of flower, with half of the standard cut away, to show what takes 

 place when the apex of the keel is liberated. The figures (also those from 423 to the 

 present), and the first account of the adaptations of Apios, were published in the Amer- 

 ioan Agriculturist in 1876. 



