The amount and

of the inherent differences which have been handed down to them from their ancestors by the factor of heredity. nature of this variation can not be discussed in detail. apply the principle directly to the potato.

We will

We may choose from a bin of potatoes of one variety 200 tubers. These tubers may appear very much alike in regard to their uniformity, size, color, depth of eyes, and the like. Indeed, from external appearances we would judge that one of these tubers was just as good as another. And this may be true so far as the desirability for present use as food is concerned. But let us test the transmission of these 200 individuals. This can best be done by use of the "tuber unit" method devised by Webber.1

Tuber Unit Method

At

In this method each tuber is cut lengthwise into four sections, cutting from the seed end in order that some of the seed-end eyes may be in each seed piece. Then the four pieces of each potato are planted in four consecutive hills. These four hills constitute the unit. The lot of 200 tubers is planted in this manner. harvest time each unit is dug separately. At that time the inherent difference which was in the original parent tubers will become apparent. Some units will yield a great deal more than others, although they are located side by side on the same soil. In Table 1 is given the yield of a series of tuber units selected at random from a breeding plot such as described above. An inspection of Table 1 shows us that there was a great difference in the yielding capacity of the individual tubers. The lowest unit yielded at the rate of 25 bushels per acre while the highest one yielded at the rate of 320 bushels per acre. Mr. PotatoGrower, from which unit would you prefer to save seed for next year's planting? Does the successful dairyman breed from the high producing or the low producing cows?

Test of Transmission

The question may arise as to the transmission of this yielding capacity. Will tubers selected from the good yielding units on

1 Webber, H. J., Plant-Breeding for Farmers. Bulletin 251, Cornell University Agricultural Experiment Station, February, 1908.

strains which have been transmitted for four bud generations. The yields in this table are based on the average yield per hill. For a better comparison the five year average is also given in bushels per acre. The five year average of the low yielding strain is 82 bushels per acre, while that of the high yielding strain is 208 bushels per acre. In Fig. 190 this comparison is

shown graphically by means of wire cylinders. The height of the column in each cylinder represents the comparative yield of that strain.