140 



The Journal of Heredity 



do know that the ]jrolificacy of any 

 variety of wheat is a complex of many 

 factors. The yield depends on the 

 climate; the chemical, physical, and 

 bioloj^ical condition of the soil; and on 

 the qualities inherent in the variety. 



Of the qualities inherent in the variety 

 the most important are the capacity to 

 develop a vigorous root system, and to 

 stool thoroughly. Other factors are 

 the average length and density of the 

 ears — the niunber of fertile florets 

 carried to each s]:)ikelet, and the average 

 size of the grain. We do not know, 

 however, as yet whether high yielding 

 and low yielding capacity behave as 

 Mendelian characters and segregate 

 as such in the second generation. 



CONCLUSION. 



During the past few years efforts have 

 l)cen made by ardent enthusiasts to 

 extend Mendel's law to all branches of 

 animal breeding, and to make it fit 

 in with our j^resent day knowledge. 

 Interesting results have certainly been 

 obtained in the cross-breeding of poul- 

 try, mice, rabbits, and polled cattle, but 

 a considerable amount of ingenuity 

 will be required to explain many of the 

 discrci)ant and discordant results ob- 

 tained with sheep and pigeons, etc. 



Whatever the future may have in 

 store in the practical ai3])lication of 

 Mendel's work to animal breeding, 

 there can be no doubt that the present 

 day breeder can, with the aid of the key 

 given by Mendel, proceed on his work of 

 i:)lant im])rovemcnt without leaving 

 much to chance. The best results will 

 follow when the individual plant is 

 regarded as being built Uj) of a number 

 of unit characters, each of which follows 

 a definite scheme of inheritance. The 

 terms dominance and recessiveness 

 should be ai)]:)lied, not to individual 

 plants, but to each of the unit characters 

 which collectively make up the organ- 

 i.sm. The schemes of inheritance of 

 many of the unit characters have been 

 worked out in detail, but there are 

 qualities of great jjractical imi)ortance 

 which require further investigation. 

 We require to know exactly what arc the 



^ Pucrinia graminis is ihc late stem-rust f)f the United States. The early oratiRe Kaf-nist of 

 the U. S. is P. rubigo-7cra. — The E(Htor. 



Similar results were obtained with the 

 inheritance of yellow rust {Pticcinia 

 glumarum) whicli does great damage in 

 England. 



Crosses Ix'tween x'arieties which were 

 immune from n'cIIow rust and Michigan 

 Bronze, a variety inordinately ])rone to 

 rust, gave a first generation crop of 

 hybrids which were as badly affected 

 with rust as Michigan Bronze itself. 

 A statistical examinaticMi of the second 

 generation plants gave 1,603 diseased 

 plants and 523 immune, or a ratio of 

 3.07:1. Apjjarently, therefore, immu- 

 nity and suscejjtibility to yellow rust 

 behave like Mendelian characters. 



It is not known to what the resistance 

 of the rust is due. Working with 

 Professor Biffcn, Miss Marryat found 

 that the rust hyjjhai are checked after 

 entering the stomata of the resistant 

 plants. Bateson points out that if the 

 resistance to yellow rust is due to the 

 presence of some anti-toxin the domi- 

 nance of suscciJtibility must be taken to 

 indicate that the formation of the 

 anti-toxin is prevented by the presence 

 of a factor in the dominant form, a 

 conclusion which may lead to definite 

 progress in the physiology of disease 

 resistance. This yellow rust (Pticcinia 

 f^ltimanim) is not the rust so frequently 

 found in Australia wheat fields. The 

 species of rust which causes so much 

 damage here is Pticcinia graminis."" 



Many of the unit characters so far 

 studied in wheat have been those which 

 are of relatively little value to the prac- 

 tical agriculturist. The color of the 

 chaff, the character of the awns, the 

 hairs on the glumes, etc., are of great 

 interest from a scientific ])oint of view, 

 inasmuch as a systematic study of them 

 will serve to throw much light (m obscure 

 problems of inheritance, but they arc of 

 infinitely less practical utility than such 

 characteristics as ])rolificacy, drouglit 

 resistance, and early maturity. 



Unf<jrtunately very little work has 

 been done in regard to these im]jortant 

 practical ])roperties, and a systematic 

 analysis of the factors on which these 

 qualities depend, and of their mode of 

 inheritance, is urgently required. We 



