AGRONOMY 



without being dusted with pollen and at the same time brush- 

 ing off upon the pistil the pollen brought from other flowers. 

 Many insect-pollinated flowers, in order to better direct the 

 attention of insects to the nectar, have various colored lines 

 and dots, called nectar guides, on the petals and sepals. In 

 addition, the petals and sepals may protect the nectar from 

 being dried up by the sun or diluted by rain and dew. In 

 some flowers the stamens and pistils are so arranged that 

 pollination may be effected by pollen 

 from their own stamens. This is called 

 self or dose pollination. Usually, how- 

 ever, the stigma and stamens ripen at 

 different times, or are so placed that 

 pollen from another flower is required 

 in order to produce seeds. When this 

 occurs the process is called cross polli- 

 nation. All the highly specialized flow- 

 ers are adapted for cross pollination, 

 and this arrangement has been found 

 to produce more vigorous and versatile 

 offspring. Wind-pollinated flowers have 

 to produce a great abundance of dry, 

 powdery pollen grains to insure that, 

 when these are intrusted to currents of 

 air, enough will find the waiting pistils to render their ovules 

 fertile. Insect-pollinated flowers, on the contrary, having 

 adopted a more certain method of transfer, do not have to 

 produce so much pollen. In some species the flower contains 

 only one or two anthers, and yet it finds this number sufficient 

 for its needs. 



In adapting themselves to insects and other agencies for the 

 transfer of pollen, flowers have become more varied than any 

 other organ of the plant. Running through all their variations, 

 however, a general plan of the flower may be discerned. In 



FIG. 72. Flower of the 

 nasturtium (Tropaeolum), 

 with two large nectar 

 guides and three " false " 

 nectar guides 



False nectar guides are 



supposed to discourage the 



visits of small insects 



