570 BOTANY OF THE LIVING PLANT 



observers, which operate in the distribution of the characters of the 

 parent forms among the progeny. The following description of one 

 sample of Mendel's experiments is based, with Professor Punnett's 

 permission, upon passages in his book on Mendelism. 



In the selection of a plant for experiment Mendel recognised that 

 two main conditions must be fulfilled. In the first place the plant 

 must possess evident differentiating characters ; and, secondly, the 

 experimental plants must be protected from the influence of foreign 

 pollen during the flowering period. In Pisum sativum Mendel found 

 an almost ideal plant to work with. The flowers are self-fertilising, 

 whilst complications from insect-interference are practically non- 

 existent. There are numerous varieties of the Garden Pea exhibiting 

 characters to which they breed true. Mendel selected a certain number 

 of such differentiating characters and investigated their inheritance 

 separately for each character. Thus in one series of experiments he 

 concentrated his attention on the stature of the plants. Crosses were 

 made between tall and dwarf races, which previous experience had 

 shown to come true to type with regard to these characters. It mat- 

 tered not which was the pollen-producing and which was the seed- 

 bearing plant ; in every case the result was the same. Tall plants 

 only resulted from the cross. For this reason Mendel applied the 

 terms dominant and recessive to the tall and dwarf habits respectively. 

 Seeds collected from the hybrid plants (F 1 generation) and sown the 

 following year, gave both tall and dwarf plants among the progeny 

 (F 2 ). Every individual was either tall or dwarf, and no intermediates 

 appeared. In one series of experiments Mendel obtained 1064 F 2 

 plants, of which 787 were tall and 277 dwarf ; that is the dominant 

 and recessive characters occurred in the second generation of hybrids 

 (F 2 ) in the approximate proportion of 3 : 1. 



In the following year seeds from the F 2 generation were sown as 

 before and produced the F 3 generation. From the seeds of the dwarfs 

 came only dwarfs, i.e. the recessive character bred true. The tall plants 

 however, of the F 2 generation now revealed themselves as being of 

 two kinds. Some of them produced seed giving rise to tall plants 

 only ; others formed seed from which sprang both tails and dwarfs in 

 the ratio of 3 : 1. The former, which evidently carried only the tall 

 character, are " pure " dominants ; the latter, which carried both tall 

 and dwarf characters, are " impure " dominants. By observation of 

 the F 3 and subsequent generations, Mendel showed that the pure 

 dominants and the recessives always bred true, resembling in this 

 way the original parents. The impure dominants, on the other hand, 



