[Chap. XXXIIl SEXUAL REPRODUCTION IN FLOWERING PLANTS 385 



are available. Some of the facts and speculations about the relation of 

 flowers to pollination by insects may be found in the books cited at the 

 end of this chapter. 



The most remarkable cases of pollination by insects are those in 

 which the plants have become dependent upon certain insects for the 

 transfer of pollen. Cross-pollination of red clover is brought about 

 primarily by bumblebees, and pollination of several of our horticultural 

 plants is accomplished mainly by honeybees. The pollination of the 

 widely cultivated yucca by the Pronuba moth is even more remarkable. 

 The female moth deposits her eggs in the ovulary of the yucca flower, 

 collects pollen from the anthers, carries it to the tip of the pistil, and 

 then pushes it down inside the tubular stigma. The young larval insects 

 which develop from the moth eggs inside the ovularv eat some of the 

 developing seeds, but not all of them. The mature moths do not live long 

 enough to see their ofi^spring, but the instinct to deposit eggs in the 

 pistil of the yucca flower and then pollinate it reappears through heredity 

 in each generation of moths. The relation of the fig to a parasitic fig 

 wasp is equally dependent, but much more complicated. 



All such cases of specialization are, of course, the result of a long 

 series of heritable variations during the millions of vears that flowering 

 plants and insects have been in existence. An enormous \arietv of 

 heritable differences in floral structures has evolved and sur\ ived, but 

 attention is usuallv directed onh^ to those that appear to influence polli- 

 nation, and to those we find desirable for decoration or for the classifica- 

 tion of plants. Those of no survixal value to the plant, or of no direct 

 value to man, are usually ignored except by a few special students who 

 find in them numerous contradictions to the claims of those who have an 

 eye onlv for useful or allegedlv useful structures. 



Pollen and hay fever. Many people suffer every vear from hay fever 

 and "seasonal asthma," caused by unusual sensitivity of the mucous 

 membranes of the eves, nose, throat, and bronchial tubes to pollen of 

 certain species of plants. The worst onslaught of hav fever is usually in 

 late summer and is caused by the pollen of plants such as ragweeds, 

 cocklebur, sagebrush, pigweeds, and thistles. Early-season hav fever 

 mav be caused by the pollen of such plants as maples, willows, birches, 

 tulips, and grasses. The number of plants to which one person or another 

 is allergic is large. 



The sufferer is frequently affected seriously onlv bv the pollen of one 

 or a few species of plants, and these can be ascertained by proper skin 



