Arrangements for Securing Union of Sexes 311 



the advantage of the cooperation of two parent plants in the produc- 

 tion of offspring. 



In my discussion of this subject I am assuming that the reader 

 ah-eady has some general knowledge of the relationship existing 

 between flowers and insects. Surely 

 there is no one whose attitude towards 

 nature is such as to lead him to read 

 thus far in this book, who has not ob- 

 served with interested attention the 

 actions of insects among the flowers 

 in a garden; and a little more watch- 

 ing will always reveal the same 

 things in the flowers of field, road- 

 side or forest. But it may be well if 

 I insert at this point, in further illus- 

 tration of our subject, a description 

 of some conspicuous examples of ad- 

 aptations to cross pollination. 



There grows commonly in Europe, 

 and sparingly in this country where it 

 has been introduced, a small upright 

 herbaceous plant called Aristolochia 

 Clematitis, whose yellow tubular 

 blossoms, an inch or so long, stand 

 upright and invitingly open when 

 ready for fertilization. It is cross 

 pollinated by small flies, which, bringing pollen on their bodies 

 from other flowers, slip easily down the tube through the 

 downward-pointing hairs (figure 111). Then, working around 

 after the nectar in the middle part of the chamber, to which 

 they are confined by other hairs in the base, they leave their 

 pollen on the stigmas (the hooked structures of the figure), 

 which soon curl back out of the way of further pollination. Im- 

 mediately the hairs in the base wither up, and the insects go there 



Fig. 111. — Flowers of Aristolochia 

 Clematitis, just before aud just 

 after pollination, the method 

 whereof is explained in the text. 

 (Simplified somewhat from Sachs' 

 Lectures.) 



