50 



BIOLOGY AND HUMAN LIFE 



Stigma 



Fig. 



When plants are transferred from one part of the world to 

 another, it sometimes happens that they fail to bear seeds be- 

 cause the particular insect upon which they depend is absent 

 in the new region. This was the case when vanilla culture 



was extended from Mexico and South 

 America to various islands in the Indian 

 Ocean. Here the plants grew luxuriantly 

 and bore many flowers, but ripened no 

 pods, or "beans" (for which the plants 

 are raised), because the insect necessary 

 for pollenation was not present (see 

 Fig. 24). 



When fig trees were first introduced 

 into California they produced large, juicy 

 fruit ; but they did not dry properly and 

 could not be prepared for shipping. To 

 get the normal fruit it was necessary to 

 find the insect that brings about pollena- 

 tion. This little wasp has a curious life 

 history which is closely tied up with the 

 fig plant. The wasp cannot complete its 

 life cycle except with the aid of the fig; 

 the fig cannot complete its life cycle ex- 

 cept with the aid of the wasp. 



A living thing cannot live by itself 

 alone, or at least not completely. In gen- 

 eral, all animals depend upon plants for 

 their means of life, and many plants de- 

 pend directly upon animals. But there 

 seems to be no advantage in being so 

 dependent upon some particular plant or 

 animal as to be unable to live without 

 Plants that are not so highly special- 

 ized, depending upon the wind for pollenation, seem to be at 

 least as well off, or at least many are quite as capable of reach- 

 ing to all parts of the earth— for example, the grasses, the 



23. Pollenation 

 by insects 



In the lady's slipper and 

 in many other flowers, 

 insects alighting on the 

 corolla crawl into the in- 

 terior, guided by the form 

 and the markings. In 

 many flowers the arrange- 

 ment of the parts is such 

 that the insect must brush 

 against the stigma in 

 going in, and against the 

 anthers in passing out. 

 As a result the animal 

 carries pollen from flower 

 to flower. Many species of 

 plants, especially among 

 the orchids, depend upon 

 single species of insects 

 for their pollenation 



it (see section 27) 



