December. 1921. 



SCIENTIFIC AGRICULTURE. 



119 



"vigorous in growth aiul iiave gained their 

 place eommereially largely because of it. 



Sterility has still another bearing upon 

 breeding work. Recent investigations have 

 shown that arrested development may take 

 place at many stages in the development 

 and functioning of the complicated sex 

 mechanism. Viewed from the genetic 

 standpoint arrested growth at any stage, 

 whether in the gamete or in the zygote, has 

 its greatest influence upon breeding re- 

 sults by eliminating factor combinations 

 which may be in many respects particularly 

 desirable. 



When pollen development is studied in 

 fruits it is found that in a large number 

 of varieties, especially those which are in- 

 terspecific hybrids, there are varying 

 amounts of aborted pollen. The outstand- 

 ing exceptions are varieties in which only 

 one species is represented. In some in- 

 stances aborted pollen is the limiting factor 

 in fruit setting either in crosses or in the 

 orchard. If the aborted grains represent 

 certain factor combinations which cannot 

 be brought to maturity and the grains 

 which do reach maturity represent those 

 which can, then it will be evident that 

 where aborted pollen occurs only a part 

 of the possible genetic combinations can 

 be made. 



Likewise in embryo development, as in 

 pollen development, arrested growth be- 

 fore maturity eliminates many possible 

 new types even in large progeny. If this 

 is a correct statement of the condition 

 found to be so general, then the limitations 

 upon breeding work with horticultural 

 material, may be much greater than has 

 been supposed. 



Considering now arrested development 

 in the zj^gote, which must of course begin 

 with the fusion of gametes which have 

 been able to mature or survive as noted 

 above, it will be clear that still further 

 elimination may take place betw^een fer- 

 tilization and seed maturity. Then, in the 

 seed bed the failure of seeds to germinate 

 results in further loss of gametes, among 

 which may be factor combinations which 

 germinate with difficulty, if at all. 



By following the formation of the game- 

 tes and zygotes through in detail it will be 

 seen therefore that many possible combina- 

 tions may be eliminated when t^iey are sup- 

 pressed or aborted in growth. The extent 

 to which this takes place will be appre- 



ciated hy those Avho have attempted breed- 

 ing Avork on a large scale and who have 

 made note of aborted pollen and pistils, 

 of the number of flowers which failed to 

 set, or the number of seeds which failed 

 to grow. 



These inherent characteristics of horti- 

 cultural material are exaggerated in in- 

 terspecific hybrids. Since these are so 

 outstanding in fruit characters in many of 

 the variet}^ lists and consequently furnish 

 what appears to be the most promising 

 combinations to make in breeding experi- 

 ments, the difficulties in breeding seem to 

 be increased accordingly. This is espe- 

 cially true in view of the fact that inter- 

 specific hybrids appear at present to be the 

 only method of obtaining varieties suit- 

 able to some localities. The origination of 

 interspecific varieties will be continued 

 therefore in spite of any handicap but with 

 reduced efficiency. 



In view of sterility and loss of vigor the 

 heterozygosity of horticultural material 

 for so many characters will render it diffi- 

 cult to develop breeding stock of known 

 genetic constitution. This will be espe- 

 cially true when the time element is taken 

 into consideration in long time crops. 

 These considerations constitute some of the 

 main obstacles to immediate progress. The 

 problem should be approached with the 

 object of studying the material at hand 

 for fundamental knowledge rather than 

 obtaining immediate practical results as 

 has heretofore been the primary object. 

 The fact should not be lost sight' of, how- 

 ever, that vegetative propagation and the 

 relative permanence of the clone counter- 

 balance in part, at least, some of the dif- 

 ficulties pointed out above. 



Breeding experiments then are not sim- 

 plified by some of the characteristics of 

 our material and the problem of the inter- 

 specific hybrid becomes even more diffi- 

 cult when these limitations are taken into 

 consideration. With the above discussion 

 in mind it may be encouraging to consider 

 briefly some of the outstanding things al- 

 ready accomplished in breeding horticul- 

 tural crops. 



We should not lose sight of the fact that 

 many varieties of outstanding merit have 

 already come from breeding attempts. 

 This is true with some fruits, as the grape 

 or plum, to a greater extent than with 



