16 ON FLOWERS AND INSECTS. [LECT. 



In those species in which self-fertilization is prevented 

 by the circumstance that the stamens and pistil do not 

 come to maturity at the same time, the stamens 

 generally ripen first. 



The advantage of this is probably connected with the 

 visits of bees. In those flowers which grow in bunches 

 the lower ones generally open first. Consequently in 

 any given spike the flowers are at first all male ; subse- 

 quently the lower ones, being the older, have arrived at 

 the female stage while the upper ones are still male. 

 Now it is the habit of bees to begin with the lower 

 flowers of a spike and work upwards. A bee, therefore, 

 which has already dusted herself with pollen from 

 another flower, first comes in contact with the female 

 flowers, and dusts them with pollen, after which she 

 receives a fresh supply from the upper male flowers, 

 with which she flies to another plant. 



There are, however, some few species in which the 

 pistil ripens before the stamens. One is our common 

 Scrophularia nodosa. Now why is this ? Mr. Wilson 

 has given us the answer. S. nodosa is one of our few 

 flowers specially visited by wasps ; the honey being not 

 pleasing to bees. Wasps, however, unlike bee's, generally 

 begin with the upper flowers and pass downwards, and 

 consequently in wasp flowers it is an advantage that the 

 pistil should ripen before the stamens. But though the 

 stamens generally ripen before the pistil, the reverse 

 sometimes occurs. Of this a very interesting case is that 

 of the genus Aristolochia. The flower is a long tube, 

 with a narrow opening closed by stiff hairs which point 

 backwards, so that it much resembles an ordinary eel- 

 trap. Small flies enter the tube in search of honey, but 



