268 TAKING UP OF POLLEN BY INSECTS, 



In the keel are concealed a style and ten stamens, all in a state of tension like 

 watch-springs. The anthers liberate their pollen very early, and it accumulates 

 in the front part of the keel. When the pulvinate wings, and through them 

 the keel, undergo pressure from above, the blunt teeth which fasten the wings 

 and keel together slip down, and both wings and keel fall with a sudden jerk, 

 whilst the stamens and style lying at the bottom of the keel spring up, throwing 

 the mealy mass of pollen into the air. In nature the object to which the pressure 

 on the wings is due is usually a largish insect, and the result is of course just 

 the same, so that the under surface of the creature's abdomen receives the dis- 

 charged pollen (fig. 274 ^). 



The pollen in these flowers being of floury or powdery consistency, a great 

 cloud of dust is emitted whenever the explosive mechanism is brought into play. 

 The same effect is produced as if the flowers exploded, and several of the plants 

 in question — as, for instance, the various species of the genus Schizanthus — are 

 called by gardeners "plants with explosive flowers". Apparatus for ejecting the 

 whole of an anther's pollen at once in a single coherent mass are of much rarer 

 occurrence. The flowers of a Brazilian shrub named Posoqueria fragrans, be- 

 longing to the order of Rubiacese, and those of a few tropical Orchids are 

 especially remarkable in this respect. The blossom of Posoqueria reminds one 

 in many ways of that of the Honeysuckle, exhibiting like the latter a corolla 

 composed of a long horizontal tube and five short limb-segments which are 

 somewhat reflexed when the flower opens. The opening takes place in the 

 evening; the corolla is white, secretes honey at the bottom, and emits at dusk 

 and during the night a pervading scent — all characteristics of a nature to indi- 

 cate that the flowers are adapted to the visits of Sphingida3. The abundant 

 honey at the base of the tube can only be reached by the tongues of Sphingidse; 

 and only these insects, e.g. Sphinx rustica, whose proboscis is 15 mm. long, have 

 been seen to visit the flower. 



The five anthers are united into an oval knob directed obliquely downwards 

 and containing the loosely-coherent pollen which escaped from the anthers before 

 the expansion of the flower. The filament of the lowest stamen possesses a 

 very considerable elastic tension acting upwards; those of the upper and lateral 

 stamens have a similar tension outwards. The insect's proboscis has only one 

 available point at which to enter the flower, and when in doing so it touches 

 one of the upper stamens at a certain spot the tension of the stamens is 

 released. The lowest stamen springs up with such violence that it hurls the 

 loosely-coherent pollen against the insect's proboscis at an angle of 50° with the 

 tube of the corolla, and with an initial velocity of about 3 mm. per second; at 

 the same time it closes the entrance to the tube. The upper and lateral stamens 

 spring at the same time to the sides, the empty anthers of an upper and a 

 lateral stamen remaining coherent on either side. About twelve hours afterwards 

 the lowest stamen extends itself again and leaves the entrance to the flower open 

 once more. If a hawk-moth, after exploding a flower in the first stage, comes 



