448 BULLETIN No. 127. [August, 



The fairly extensive recorded history of 5\ tuberosum for three 

 hundred years is interesting to the student of changes under do- 

 mestication, in that the changes which have taken place in the plant 

 ;have been almost entirely in the selected part, the tuber. Even the 

 change in the tubers seems to be largely a matter of lesser numbers 

 .and larger size per plant. The minor distinctions that have sepa- 

 rated modern varieties have been in the main those of shape and 

 color. Slight differences, provided the variety is a fairly high 

 yielder, have been the cause of a large number of names, but in the 

 United States, at least, the productions of scientific breeders com- 

 paratively few in number, furnish practically the entire crop of the 

 country. 



Methods of hybridizing the potato are comparatively simple, 

 but the variation of the varieties in their ability to furnish viable 

 pollen has been a source of disappointment with a great many 

 varieties possessing other particularly good characters. A problem 

 here is to find means of stimulating the production of pollen in 

 these varieties. This matter is also of special theoretical interest 

 in its bearing on the production of sex. 



The method of propagating the potato has seriously increased 

 the number of errors .in making comparative field tests during the 

 establishment of varieties. The fact that the seedlings must be 

 grown several years in the comparison test before making definite 

 selections, makes care in this respect still more important. The dis- 

 covery of characters in the young plant which are correlated with 

 characters in the tuber would be of great advantage in shortening 

 the time of their comparison before final rejection of the unfit. 

 Some work has already been done in this direction. 



Possible methods of improvement are three : 



1. The crossing of desirable plants and raising of many seed- 

 lings under controlled conditions. 



2. Selections of the most desirable fluctuations among the plants 

 and tubers of a variety. 



3. Selection of discontinuous variations^ and a study of ways 

 of causing them, a possible example being the so-called graft-hybrid. 



That advance is possible by the first method is admitted from 

 the results of its use by the best breeders. More data as to the best 

 details for work in use of this method, however, are very desirable. 

 Whether results can be obtained by the second method has not been 

 definitely proved. The data here shown which do point to the af- 

 firmative, are not conclusive. Isolated cases of improvement might 

 reasonably be explained as due to mutations. 



