SOME OP THE PRINCIPLES OF PLANT- BREEDING 



61 



sporting rather than being simply representative 

 of the cumulative result of the selection of slight 

 individual variations. The sugar-beet furnishes an 

 interesting illustration in this direction. It will be 

 remembered that Louis Vilmorin started the selec- 

 tion of sugar-beets for richness in sugar, between 

 1830 and 1840, selecting first by means of specific 

 gravity, the method being to throw the beets into 

 solutions of brine strong enough so that the great 

 majority of them would float, the few which sank 

 being of greater specific gravity and presumably 

 of greater sugar content. Considerable improve- 

 ment was produced by this method. About 1851 

 the method of chemical analysis was introduced to 

 determine the exact sugar content. At this time 

 the sugar content was found to vary from 7 to 14 

 per cent, and in the second generation of selection 

 individuals with 21 per cent of sugar were found. 

 The selection based on sugar content, using the 

 beets highest in sugar content as mothers, has been 

 continued regularly since that time, and the indus- 

 try has come to rely entirely on careful selection 

 for high sugar content. It would be expected that 

 under these conditions the sugar content would 

 have increased sufficiently so that the selected 

 plants could be considered a different race or 

 strain. Yet, after fifty years of selection, the 

 highest sugar content found is only about 26 per 

 cent, and this in a very few instances, seldom over 

 21 per cent being found. At the present time many 

 thousand analyses are made every year, so that 

 abundant opportunity is afforded to find individuals 

 producing a high sugar content. On the contrary, 

 when Vilmorin's work was started the determina- 

 tion of sugar content was by very laborious meth- 

 ods, and was limited to comparatively few indi- 

 viduals. It is not improbable that if Vilmorin had 

 been able to make analyses of the sugar content in 

 many thousands of roots he would have found cer- 

 tain individuals producing as high as 26 per cent. 

 The inference from this illustration would be that 

 the limitations of the variation within the race 

 have not been surpassed as a result of selection. It 

 may be argued, however, that in this case we are 

 dealing with a physical impossiblity, as it is clearly 

 evident that it would be impossible for a plant to 

 produce a root containing a proportion of sugar 

 beyond a certain percentage, and it is thus possi- 

 ble that 26 per cent, or thereabouts, represents the 

 maximum. 



It must be admitted that in many cases we have 

 an apparently cumulative effect of selection, and it 

 seems almost impossible to draw the line between 

 improvements created by continuous selection of 

 slight individual variations within the race or the 

 selection of those plants which are mutations. In 

 the case of the gooseberry, tomato and many other 

 plants, the fruits have been increased in size grad- 

 ually, until they are now four to eight times that 

 of the original wild fruits. Much of this increase 

 in size has of course been accompanied by hybridi- 

 zation between different wild species and different 

 races of the same species which have been mixed 

 together, yet it is a cumulative gain in size, as 

 none of the wild types ever produce fruits nearly 



so large as those of the cultivated races that have 

 been developed. Practically the entire development 

 of the tomato has taken place within the memory 

 of men now living, and in this case the develop- 

 ment has not been accompanied by hybridization 

 of different species but by the selection of different 

 races within the species and the hybridization of 

 these races. One of the experiments conducted by 

 DeVries with corn is of interest in this connection. 

 This experiment was undertaken for the purpose of 

 increasing the number of rows of kernels on the 

 ear. The corn used in the selection averaged 

 twelve rows at the time the selection began. After 

 seven generations of selections from ears which 

 bore the largest number of rows, the mean was 

 raised to twenty rows. In the first year of the selec- 

 tion the variation in number of rows ranged from 

 8 to 20. In the seventh generation of selection 

 the variation in number of rows ranged from 12 to 

 28. This shows clearly the increase in the number 

 of rows and the development of an apparently new 

 race by simple selection. However, when the selec- 

 tion was discontinued the improvement or new 

 character was soon lost. 



The majority of new races produced as a result 

 of selection are due, without much ,5oubt, to the 

 choice of mother plants showing marked variations 

 which. we would term mutations, and which are 

 referred to by gardeners ordinarily as sports. 

 In reviewing the history of cultivated varieties, 

 one is surprised at the large number of varieties, 

 which have had their origin in this way. Many of 

 our apple, pear and peach varieties are simply 

 accidental seedlings which have sprung up in fence- 

 corners or door-yards, and a number of our wheat, 

 tobacco and cotton varieties have been developed 

 by selection from certain individual plants that 

 have attracted attention because of the exhibition 

 of superior qualities. It is probable that a large 

 number of these accidental and selected varieties, 

 particularly in the case of apples and pears, are 

 really the results of accidental hybridization, and 

 the same may be true of many wheat, corn and 

 cotton varieties. Yet there are many cases in 

 which the mutations or extreme variations cannot 

 be traced back to hybridization. In the production 

 of the Cupid sweet-peas, for example, the first 

 small dwarf plant of this type was found growing 

 in a row of the Emily Henderson, which is one of 

 the normal climbing forms of the sweet-pea. At 

 that time no other dwarf type of the sweet-pea was 

 known, and this variation, therefore, cannot be 

 accounted for as due to hybridization with some 

 other dwarf form. It is impossible to account for 

 these striking variations which sometimes occur, 

 but it is important that all plant-breeders be on 

 the lookout for the occurence of new types and 

 variations of this sort. 



The writer has been asked frequently whether 

 it is possible to select a plant so highly that 

 it will not revert to the original mother type. 

 Experience wduld indicate that when the mother 

 plant from which the selection is made is a true 

 mutation, like the sweet -.pea mentioned above, 

 the type will maintain itself even after the 



