Chap. 20 THE PHYSICAL BASIS OF HEREDITY 391 



Before the flower is quite developed the bud is opened and the stamens con- 

 taining the pollen are removed (Fig. 20.2). Then pollen from another plant 

 is placed on the pistil through which the male cells make their way to the eggs. 

 Mendel chose plants of two pure-line varieties, that is, one in which for 

 several generations the plants had been tall and another in which they had 

 been dwarfs, terming these the parental generation (P). He cross-pollinated 

 flowers from these two parent stocks. All of the resulting hybrids were tall 

 plants, the First Filial or Fi generation (Fig. 20.3). The dwarf character 

 had disappeared. However, when the plants of this (Fi) generation were 

 self-pollinated and another generation (F^) was produced, the dwarf ness 

 termed the recessive character turned up again. Not only that, but it appeared 

 in a regular and predictable ratio of three tall, termed the dominant characters, 

 to one dwarf, the recessive. 





stamen 



Pollen grain 



Pollen tube 

 Sperms 



Ovule 

 Ovary 



B 



Fig. 20.2. Flower of garden peas, the subjects of many of Mendel's experiments. 

 A, diagram of the flower with the petals, consisting of standard, wings and keel, 

 separated to expose the pistil and stamens. The boat-shaped keel and the wings 

 naturally close tightly around the pistil and stamens insuring self-pollination. B, 

 diagram of the pistil and Stamens of the pea showing the pollen tube that grows 

 downward carrying the sperm that fertilizes the egg. Other sperms unite with nuclei 

 in the ovules (not shown) to produce the nutritive part of the seed. (Courtesy, 

 Colin: Elements of Genetics, ed. 2. Philadelphia, The Blakiston Co., 1946.) 



