492 WHITE— STUDIES OF INHERITANCE IN PISUM. 



3. Changes in pea varieties due to mutation will also be dis- 

 cussed under the heading of mutation. Mutations, in the sense 

 used in this paper, are relatively sudden, abrupt variations in a 

 strain of plants which has bred true for more than two generations 

 in the same environment. These variations remain comparatively 

 constant in succeeding generations and form the basis of a new 

 strain or variety. Such characters in peas as white flower color, 

 lack of parchment in the pod, yellow foliage, and absence of ten- 

 drils have, so far as we now know, resulted from mutation. Muta- 

 tions are comparatively comrnon in some organisms and rare in 

 others. Morgan and his students (61) have records of over 200 

 character changes in the fly, Drosophila, resulting from mutation. 

 In peas, this phenomenon, judging by the records, is comparatively 

 rare. Any type of character may be altered or replaced by muta- 

 tion, the change occurring either as a small or as a large variation. 



The Material and the Technique. 



" The value and utility of any experiment," says Mendel, " are 

 determined by the fitness of the material to the purpose for which 

 it is used." Mendel (60), Correns (15), and Lock (53) have each 

 expressed themselves strongly regarding the exceptional value of 

 peas as material for the experimental study of heredity. The fact 

 that they possess easily recognized constant differentiating charac- 

 ters, flowers which ordinarily are self-fertilized, and are capable 

 of giving perfectly fertile F^ hybrids was the chief reason that 

 Mendel chose them. Mendel's reasons coupled with certain other 

 facts, such as the direct economic value of the results, and the quick 

 maturity of the plants, have led to their choice for the present series 

 of studies. 



Planting. — Peas are easily grown, and mature as many as three 

 generations a year if both greenhouse and field plots are used. 

 They are sown the ist of April in this latitude, or earlier if prac- 

 ticable because the late varieties mature poorly under our summer 

 conditions. Wrinkled seeded varieties rot before germination more 

 easily than round seeded varieties. The peas are sown in rows, 

 from 10 to 15 cm. apart in the row, the rows being 1.2 meters or 

 more apart. Only undiseased plump seed are planted, unless there 



