340 . GENETICS IN RELATION TO AGRICULTURE 



says, "the indications are that from a practical breeder's standpoint 

 permanent improvement in pure lines in small grains, if possible, is 

 certainly not rapid or apt to be very marked." He also suggests that 

 much more rapid progress could be made by isolating pure lines from 

 mixed populations and combining the desirable characters of these lines 

 by hybridization. 



Other crops in which the method of selecting pure lines is applicable 

 are oats, barley, peas and beans. Notable improvement has been made 

 in oats by this method at the Svalof, Cornell University and Maine 

 Experiment Stations. The general method of procedure at the Maine 

 Station is indicated by Surface and Zinn in their bulletin on pure line 

 varieties of oats (see p. 371). The pure lines finally retained came 

 from only three varieties, viz., Banner, Irish Victor and Imported Scotch. 

 It is noteworthy and consistent with Mendelian principles that the 

 physiological characters which result in higher yield are not necessarily 

 associated with morphological characters in the plant or grain. Similar 

 results with winter-resistant barleys have been reported by Spragg of 

 the Michigan Station. The practical importance of the selection of 

 pure lines as one phase of a complete system of breeding as practised 

 with autogamous species is given further attention in Chapter XXI. 



Selection which is to result in the isolation of the most superior 

 genotypes must begin with individual plants. In dioecious and self- 

 sterile plants this method is inapplicable. Here the breeder must begin 

 with phenotypically similar individuals and continue inbreeding of simi- 

 lar plants for several generations in order to isolate approximately uniform 

 strains. The earlier improvement of the sugar beet was accomplished 

 by mass selection. But in recent years the producers of commercial 

 seed have introduced a system of line selection. According to Briem, 

 reliable seeds cannot be obtained by selection in the lump, nor from a 

 single generation of mother beets followed by the cultivation of seed roots. 

 An individual selection must be made the characteristics of which are 

 assured by testing for three generations. That is to say, since the beet 

 is a biennial 6 years are required to obtain seed of guaranteed quality 

 for the seed roots and another 2 years must pass before the market 

 product is ready. Briem's opinion is in harmony with Pritchard's con- 

 clusion that continuous selection is not an efficient method of sugar beet 

 improvement and that the improvement of the past is the result of 

 isolating mutations (see p. 369). 



In emphasizing the importance of finding the best genotypes within 

 a chosen species or variety the usefulness of mass selection should not be 

 overlooked. It is frequently the first or only practicable step to take in 

 purifying a commercial variety. The so-called "running-out" of varie- 

 ties can be prevented by reasonable care to avoid mixing seed and by occa- 



