SOME PRINCIPLES OF PLANT EVOLUTION 491 



Characters which behave in this way are called unit characters. 

 Mendel studied the relation of other unit characters of the pea, 

 and found that smooth seeds are dominant to wrinkled seeds, 

 colored are dominant to white, yellow color dominant to green, 

 etc. Where two unit characters in each parent are contrasted 

 the result is still more interesting because it results in the pro- 

 duction of two new forms. For example, he crossed tall yellow 

 peas with dwarf green peas. The result was that in the first 

 hybrid generation all were tall yellows. But in the second 

 hybrid generation they split up with the following result. For 

 every sixteen plants there were nine tall yellows, three dwarf yel- 

 lows, three tall greens, and one dwarf green. It will be seen that 

 the dwarf yellows and tall greens are new forms, and successive 

 generations show that these can be extracted and grown as pure 

 forms. This is a very important discovery for it enables the 

 plant breeder, after he has determined by experiment what the 

 unit characters of his plants are, to combine certain desirable 

 characters in a single form (paragraph 679). 



662. Mutation. Mutation is the term applied to those 

 sudden variations, not due to cross fertilization, in which the new 

 form is so unlike its parent that it is regarded as a new species, 

 often termed an elementary species. The new forms arising in 

 this way by a leap or bound as it were are called mutants. The 

 phenomenon is sometimes spoken of as discontinuous variation in 

 contrast with fluctuating variation. Mutations were first thor- 

 oughly studied by De Vries in one of the evening primroses 

 ((Enothera lamarckiana\ which may be taken to illustrate this 

 type of variation. This is a large flowering evening primrose. It 

 is supposed to have been native to North America since speci- 

 mens occur in several herbaria collected in some of the southern 

 states. It was introduced into Europe where it is grown in 

 gardens, and as an escaped plant it also grows wild there in a 

 number of places. De Vries observed it growing wild near 

 Amsterdam in Holland. Growing with Lamarck's evening prim- 

 rose ((Enothera lamarckiand) were several other primroses which 

 could be recognized as different because of the form of the 



