MENDEL'S LAWS OF HEREDITY 233 



Mendel's experiments. — What Mendel sought to discover was the 

 law of inheritance in hybrid varieties, and he selected for experiment 

 the edible pea {Pisum sativum). The trial plants, he says, must 

 possess constant differentiating characters, and must admit of easy 

 artificial pollination; the hybrids of the plants must be readily fertile, 

 and readily protectable from the influence of foreign pollen. These 

 conditions were afforded by peas, and twenty-two varieties or sub- 

 species of pea were selected, which remained constant during the eight 

 years of the experiments. Whether they were called species, or sub- 

 species, or varieties, is a matter of convenience; the names Pisum 

 quadratum, P. saccharatum, P. umbellatum, etc., do in any case repre- 

 sent groups of similar individuals which breed true inter se. It should 

 be noted that these peas have the particular advantage, for experi- 

 mental purposes, that they are habitually self -fertilised — in North 

 Europe, at least. 



In studying the different forms of peas, Mendel found that there 

 were seven differentiating characters which could be relied on: 



1. The form of the ripe seeds, whether roundish, with shallow 

 wrinkles or none, or angular and deeply wrinkled: 



2. The colour of the reserve material in the cotyledons — pale 

 yellow, bright yellow, orange, or green ; 



3. The colour of the seed-coats, whether white, as in most peas 

 with white flowers, or grey, grey-brown, leather brown, with or with- 

 out violet spots, and so on; 



4. The form of the ripe pods, whether simply inflated, or con- 

 stricted, or wrinkled; 



5. The colour of the unripe pods, whether light or dark green, or 

 vividly yellow, this colour being correlated with that of stalk, leaf- 

 veins, and blossoms; 



6. The position of the flowers, whether axial or terminal; and 



7. The length of the stem, whether tall or dwarfish. 

 Mendel's results; the Law of Dominance. — Having defined the 



differentiating characteristics of the varieties, Mendel proceeded to 

 make crosses between these, investigating one character at a time. 

 Thus, pollen from a pea of the round-seeded variety was transferred 

 to the stigma of a pea of the angular-seeded variety, the stamens of the 

 artificially pollinated flower being, of course, removed before they 

 were ripe. The same was done all along the line. 



What was the result in the hybrid or cross-bred offspring ? It was 

 found that they showed one of each pair of contrasted characters, to 



