CROSS-FERTILIZATIOX OF PLANTS. 267 



discussion soon after its first appearance, some one brougtit a flower 

 from Madagascar which had a nectary measuring seventeen inches. 

 Some were inclined to ridicule Darwin's theory and ask him to produce 

 the insect having a tongue long enough to reach the sweets at the bot- 

 tom. Though no such an insect was known, Darwin readily accepted 

 the challenge and declared that he would stake his theory on the propo- 

 sition that somewhere there was an animal capable of reaching it, else 

 there would have been no such development. His critics were much 

 disconcerted soon afterwards by the finding of just such a moth as Darwin 

 had said there must be. 



Let us consider some of the devices for preventing waste of pollen 

 in insect fertilized flowers. We can readily see that after a plant has 

 specialized to such an extent as to secure transfer of its pollen by certain 

 flying insects only, that it may be necessary to arrange some means to 

 keep out a large class of crawling insects, like the ants, which might 

 seek to take advantage of the food and at the same time be of no use to 

 the flower. They would not be adapted to transfer pollen to another 

 plant in good order. The nasturtium for instance has numerous bristling 

 hairs that bar the way to the nectary. The snow-berry has a perfect 

 ball of cotton over its sweets. Some plants as the sunflower have such 

 spiny, hairy stems as to discourage creeping visitors. Other plants 

 secrete sticky gums which act very much in the same way as hairs, in that 

 they are a serious impediment to insect travel. Some plants, like the 

 milkweed, have smooth, waxy stems which are easily punctured by the 

 sharp claws of a climbing insect. When the plant is injured the sticky 

 milk will flow out and one can readily understand how disgusted an ant 

 would soon become vv'ith such a plant. The rubber plant has sticky sap 

 for the similar purpose of self protection. 



Numerous schemes for prevention of visits by any other than the 

 favored guest might be cited. The nectary is often located in long spurs 

 where only long tongued insects can reach it. The columbine has five 

 such spurs. The common flies and bees cannot disturb this flower. It 

 reserves its nectar for certain long tongued insects. The bumble bee 

 sometimes thwarts nature's scheme by alighting on the outside of the 

 flower, and cutting a hole for a back door, as it were, drains the nectaries 

 dry without having touched the essential parts of the flower. If this 

 ingenious device of the bumble bee should become universal among bees 

 it might have a serious effect upon the plant's survival. It would then 

 have to depend upon allies of the butterfly order, who carry no knives, to 

 make the first call. 



One of the most astonishing tricks in the plant world is played by 

 the blue flag or iris. One must examine a flower and find the parts to 

 fully appreciate the situation (Fig. 29.) At last you will find the false 

 honey guides running down the interior of the flower and Needham re- 

 ports that many insects seem to make the very natural mistake of probing 

 down the center of the flower as indicated by the guides, for the nectar. 

 They find nothing and go away no doubt with a poor opinion of the flag. 



A bumble bee, for whom the fiower seems especially designed, alights 



