MENDEL'S EXPERIMENTS 167 



certain definite characters in domesticated peas were trans- 

 mitted when varieties were crossed. He chose plants in 

 which particular characters bred true under normal circum- 

 stances when left to themselves. Peas are usually self- 

 pollinated, that is to say, the ovules in the flower are 

 pollinated from the same flower or the same plant. It is 

 easy in such a case to ensure cross-pollination artificially, 

 and to prevent any possibility of self-fertilisation by removing 

 the stamens of the artificially pollinated flower before they 

 are ripe. Mendel chose very distinctive characters in the 

 pea-plants in which he produced crossing by artificial pol- 

 lination, and the differences were differences in the same 

 character. For instance, he took the length of the stem 

 of the plant whether it was long or short, that is to say, 

 whether the plant was a tall or a dwarf variety. Other 

 characters were the form of the ripe seeds, round and smooth, 

 or angular and wrinkled ; the colour of the seeds, plain or 

 spotted, green or yellow. To make the results of his experi- 

 ments clear, it is best to take only one of these characters, 

 or rather one pair of characters tallness and dwarfness of 

 the plant. All the plants produced from the seeds of the 

 first cross between a tall and a dwarf plant were tall. These 

 hybrids were self-fertilised, and the plants grown from these 

 differed from each other in that some were tall and some were 

 dwarf. Out of over a thousand plants the average propor- 

 tion of tall to short plants was as three to one. These plants 

 were again self-fertilised. The dwarfs produced nothing but 

 dwarfs, and this apparently went on for many generations, 

 the tall character never reappearing in them. Of the pro- 

 geny of the tall plants, however, one-third produced tall 

 plants only in succeeding generations. The other two-thirds 

 produced again one-quarter dwarf plants and three-quarters 

 tall plants. The dwarf plants continued to produce dwarf 

 plants indefinitely, while the tall plants broke up again into 

 the same proportions of plants that produced only tall 

 plants, and others that produced the same proportion of 

 tall and dwarf plants. The following diagram illustrates- 



