124 PLANT GROWTH 



SO important and Mendel's proof of them so clear and so 

 carefully worked out that it would be well worth our while 

 to examine Mendel's experiments in detail. Gregor Johann 

 Mendel was an Austrian monk living at a monastery at 

 Briinn (Plate VII) which has since become a Czechoslo- 

 vakian and at present a German town. He did his work in 

 the 1850's and 1860's publishing his results in 1866. He was 

 slow, careful, analytical, and did not jump to conclusions. 

 His success where others had failed lay entirely in his ana- 

 lytical procedure. Instead of considering all the differences 

 between two varieties when he crossed them, he considered 

 just one difference at a time. The plants he worked with 

 were garden peas. He selected them because they are nor- 

 mally self-fertilized, are easy to cross, and he had a number 

 of varieties which were easily distinguished from each other. 



Mendel did not plunge immediately into haphazard 

 crossing experiments. First he selected his material care- 

 fully. He obtained a number of varieties of peas from seed 

 houses and grew them. Those varieties which were not uni- 

 form he discarded. He carefully selected varieties which 

 differed from one another by characters which were easily 

 recognizable. Finally, since he considered only one charac- 

 ter at a time, he selected his varieties by pairs contrasting 

 different expressions of the same character. For instance for 

 the character, height, he selected two varieties and only two. 

 One of the varieties was tall, the other dwarf. For the char- 

 acter, shape of seed, he selected two varieties, one with 

 smooth seeds and one with wrinkled seeds. Thus, in all he 

 selected seven pairs of varieties differing in seven different 

 ways. His first seven experiments consisted of crossing the 

 members of the seven pairs with each other. 



Mendel's first experiment was to cross a variety of tall 

 peas (70-80 inches) with a variety of dwarf peas (8-18 

 inches) . The progeny of the cross were all tall. It made no 

 difference whether the tall variety was used as a male or 

 female parent. The first generation resulting from the cross 



