IMPROVEMENT AND BREEDING OF WHEAT 413 



able to treat these as separate hereditary units, which can be added or 

 removed at will. 



Moreover, he is able to combine in one plant many of the characters 

 possessed by two or more different varieties, and from a knowledge of the 

 established Mendelian ratios the selection of desirable forms which will 

 breed true is simplified and rendered more certain. For example, from 

 a red-chaffed, red-grained wheat and a white-chaffed, white-grained 

 variety it is easy to obtain both white-chaffed red-grained and red-chaffed 

 white-grained individuals, and plants constant to these new combinations 

 of characters can be selected with certainty from the second generation of 

 the hybrid's descendants. 



By crossing any of the forms just mentioned with plants having 

 glabrous or pubescent glumes, these latter characters can be introduced 

 into the progeny of the hybrids, and, similarly, the bearded or beardless 

 characters can be added to plants in which they are absent. 



In the chapter on hybridisation (Chap. XXV.) the mode of inheritance 

 of many of the different characters of the wheat plant is indicated so far 

 as it has been determined, and the knowledge can be utilised for the pro- 

 duction and breeding of plants in which are present any combination of 

 the separate characters there discussed. 



Unfortunately, colour of chaff and grain, presence and absence of awns, 

 and most of the characters whose inheritance has been clearly established 

 are of no economic importance. 



The character which above all others it is desirable to control and 

 utilise is the grain-yielding capacity of the plant, but this, like almost all 

 quantitative characters, either does not Mendelise or is beyond present 

 Mendelian analysis, and its manifestation is so greatly influenced by so 

 many external conditions that its inheritance is obscured. 



Other characters which it is desirable to improve on account of their 

 bearing upon yield are length of growing-period, resistance to drought, 

 frost, and lodging, and immunity to diseases, all of which are of a highly 

 complex nature, and their inheritance in terms of Mendelian factors still 

 uncertain. 



Whatever methods are adopted for the improvement of wheats, it is 

 essential that a clear conception should be obtained of the ideal towards 

 which the plant-breeders' efforts are directed. This is especially neces- 

 sary since the aims of the breeder vary in different countries ; the wheat 

 which best suits the English farmer is useless in Central Europe, India, or 

 Australia, and vice versa. 



Moreover, real progress can only be made by those who are thoroughly 

 familiar with the morphological details and variability of the wheat plant, 

 and the raising of improved kinds adapted to the requirements of the 

 grower to be successful must be carried on in the country in which the 



