CHAPTER TWENTY-SIX 



SEXUAL REPRODUCTION IN FLOWERING PLANTS 



The most important fact that should be associated with the 

 flower is that in it occurs the sexual reproduction of the plants. 

 Sexual reproduction takes place by the union of two specialized 

 cells, the gametes. One of these cells is called the male gamete, or 

 sperm, and the other the female gamete, or egg. When these cells 

 unite they form a single cell called a zygote, or fertilized egg. This 

 process of sexual union, or fertilization, is the first step in the 

 development of nearly all plants, and every individual plant 

 normally starts as a single cell, no matter how complicated it 

 may be at maturity. The flower is a complicated structure in 

 which the development of the gametes, fertilization, the forma- 

 tion of the zygote, and its further development takes place. 

 These several steps ending with the seed are described in the 

 following paragraphs. 



Pollination. If the anthers of a lily or nasturtium are ex- 

 amined, a fine yellow powder will be found which under a micro- 

 scope appears as a multitude of small grains. This is pollen. In 

 the production of seed it is necessary that the pollen grains be 

 carried to the stigma, and this transfer is called pollination. In 

 some plants the pollen merely falls by gravity on the stigma. 

 Wheat and oats are examples of plants that are pollinated in this 

 way. In other plants, like the pines, elms, birches, oaks, rye, 

 and corn, the pollen is carried by the wind. It is an interesting 

 fact that the stigma of wind-pollinated flowers are usually 

 roughened by hairs, which probably make them more effective 

 in holding the pollen. 



The pollen of most plants with conspicuous flowers is carried 

 by bees, flies, butterflies, or moths. As the body of one of these 

 insects is rough or hairy, pollen grains become attached to it when 

 the insect enters a flower. Then when the insect passes to an- 

 other flower, some of the pollen from the first flower is brushed 



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