158 ORGANIC EVOLUTION 



to plant, as well as from blossom to blossom of the same 

 plant, cross-fertilization between plants must frequently 

 result. 



Aristolochia sipho illustrates another method of secur- 

 ing cross-fertilization through insects. The flowers of this 

 species are in the form of a bent tube with a flaring end, 

 something like a trumpet (Plate 90, A]. Flies enter the 

 opening of this tube, but find their egress prevented by a 

 mass of hairs (Plate 90, B] which fills the tube, pointing 

 toward its base, allowing the flies to enter but not to 

 depart. The stigma of these blossoms is a large top-shaped 

 structure, nearly filling the base of the tube. Behind it, 

 and inaccessible to the flies, are the three biscuit-shaped 

 anthers with their pollen. The swollen stigma shrinks as 

 the flower grows older, and if the flies which have entered 

 have brought pollen with them and have fertilized the 

 stigma its shrinking is hastened. After the stigma has 

 shrivelled, the flies, as they wander about their prison, can 

 reach the pollen and will become well dusted with it. Now 

 the hairs which have prevented their departure dry and 

 shrivel and the flies are set free to seek another blossom 

 and fertilize its ovules. 



Each of these general methods of securing cross-fertili- 

 zation which we have illustrated is used by a considerable 

 number of plants, and there are scores of other devices to 

 which we have not space to refer. Many of these are vividly 

 described, with good pictorial illustrations, in Kerner's The 

 Natural History of Plants, the English translation of which, 

 by Oliver, is published by Henry Holt & Company. 



Enough has been said to emphasize the importance to 

 the plant of insect visits. We have seen that by the secre- 



