16 ON FLOWERS AND INSECTS. [LECT. 



There are, however, some few species in which the 

 pistil ripens before the stamens. One is our common 

 Scropkidaria 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 bees, 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 

 from the direction of the hairs it is impossible for them 

 to return. Thus they are imprisoned in the flower ; 

 until the stamens have ripened and shed their pollen, 

 by which the flies get thoroughly dusted. Then the 

 hairs of the tube shrivel up, thus releasing the prisoners, 

 which carry the pollen to another flower. 



Again, in our common Arums the Lords and Ladies 

 of village children the well-known green leaf incloses 

 a central pillar ; near the base of which are arranged a 

 number of stigmas (st in the accompanying Fig.), and 

 above them several rows of anthers (a). It might be 

 supposed therefore that the pollen from the anthers 

 would fall on and fertilise the stigmas. This, however, 

 is not what occurs. In fact the stigmas come to matu- 

 rity first, and have lost the possibility of fertilisation 

 before the pollen is ripe. The pollen must therefore be 

 brought by insects, and this is effected by small flies, 



