BULLETIN No. 127. [August, 



5. INHERITANCE OF CHARACTERS IN TUBER 

 SELECTION 



THEORY 



We do not mean by this term the inheritance of the characters 

 in future sexual crosses, but the transmission of selected variations 

 from year to year by tubers. A consideration of this question is 

 of practical importance to the potato grower. The potato breeder 

 may still continue to make crosses and originate varieties, but in 

 such work he is and must be a specialist. His work can never be 

 undertaken with profit by the average grower, to improve his stock. 

 On the other hand, if there is a possibility of selecting and propa- 

 gating favorable fluctuating variations and their accumulation for 

 the betterment of the variety, such work can be undertaken with 

 success and profit by the farmer. 



It is common knowledge that during the first few years the 

 progeny of a sexual cross in potatoes is quite variable. These 

 variations may be arbitrarily divided into two classes : First, those 

 variations that seem to be due directly to slight differences in en- 

 vironment, such as shape, size and yield of tubers, and vigor of 

 growth and amount of foliage in plants; second, variations that 

 are much rarer and that seem to be of a more nearly botanical char- 

 acter as those of color tubers, length of life of plant, and amount 

 of blossoms and production of seed. Variations, in the after life 

 of the variety are said to become less common, that is, the type of 

 the variety is said to become fixed. This appears to be true from 

 general observations of potato seedlings, and it might partially be 

 explained by the fact that each year the plants are subject to rigid 

 selection to a certain type. // these fluctuations are transmitted, 

 the plants dealt with in subsequent years are a selected and not a 

 general race. But when unselected it is probable that there is 

 a lessening variability with advancing age, even when the physio- 

 logical vigor of compared plants is kept the same. Vernon (99 p. 

 184) showed conclusively that for low forms of animals as the sea 

 urchin that the "Permanent effect of environment on the growth of 

 a developing organism diminishes rapidly and regularly from the 

 time of impregnation onwards." A little later De Vries (26) enun- 

 ciated practically the same law for plants. He concludes: i. The 

 younger the plant, the greater is the influence of external condi- 

 tions on its variability. 2. The nutrition of the seed when develop- 

 ing on the mother plant has (at least very often) a greater influ- 

 ence on the variability than during germination and growth. 



