1 88 GROWTH AND WORK OF PLANTS 



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 

 w r ith 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. 



302. Landing places for insects. Many of the flowers 

 which attract insects are irregular (those with bilateral symmetry) 

 and some portions of the flower are especially adapted to serve as 

 a landing place. This is often the lower lip of the flower, or one 

 or more of the lower petals (where more than one petal they are 

 often consolidated) . The keel of the papilionaceous flowers, the 

 lower lip of bilabiate flowers, the lower petal of the canna flower, 

 and the labellum of the orchids, are examples. In the violet the 

 insect rests on the lower petal while extracting the nectar from 

 the nectary in the spur. 



303. Honey guides. Some flowers have "honey guides," 

 bright-colored lines on the petals facing the insect as it alights, 

 which lead down to the nectary. 



304. Flower structures suited to the visits of special 

 insects. Flowers with long spurs, which are formed from the 

 prolongation of one or more sepals or petals into a tube, prevent 

 most insects, except those with a long proboscis or sucking tube, 

 from obtaining nectar. The sphinx moths and humming birds 



