GROWTH AND WORK OF PLANTS 



CROSS-POLLINATION BY INSECTS. 

 287. How insects are attracted to flowers. In order to 

 secure cross-pollination through the aid of insects, the insects 

 must be lured or attracted to the flowers for some highly prized 

 food. Provision of food is made in the nectar which is developed 

 in special nectar glands, or nectaries, in these flowers. The odor 

 from flowers probably has more influence in attracting insects to 

 them than anything else. This is very striking in the case of 

 night-blooming flowers, which usually have strong fragrant odors 

 and attract the moths which fly at night. In addition most 

 flowers adapted for cross-pollination by insects have showy parts, 

 which by their size and color attract the insects, and stand for 

 them as a sign of those flowers which produce nectar. In the 

 larger number of cases the petals are the showy parts of the flower. 

 Sometimes it is the sepals, especially in the apetalous flowers, 

 as in the marsh marigold, etc. In other cases the bracts of the 

 flowers are colored and showy, as in the flowering dogwood. In 

 many composite flowers it is the ray flowers which are showy 

 and serve to attract the insects. In many cases these ray flowers 

 are neutral, or do not develop seed, so that their sole function is 

 to attract insects, while the inconspicuous disk flowers provide 

 the nectar and produce the seeds. The neutral showy flowers 

 in the cranberry tree or wild guelder rose (Viburnum opulus) are 

 exterior to the perfect inconspicuous flowers. This massing of 

 flowers into flower clusters, heads, etc., is of great advantage 

 since the flowers are made more conspicuous, the odors are more 

 centralized, and, as in the heads of composites, the insects crawl- 

 ing over the head cross-pollinate rapidly a great many flowers. 

 According to Lubbock, flies are mostly attracted to the flowers 

 with the duller colors, as brownish, dark purple, dull yellow, or 

 greenish flowers. Some flowers have carrion odors which also 

 serve to attract flies. Butterflies and bees are attracted by the 

 bright colors, as red, blue, violet. Experiments seem to show 

 that insects cannot see the form of objects distinctly at distances 

 greater than four to six feet, but the colors of objects can be seen 

 at a greater distance. 



