128 Glume- Length and Grain- Length in Wheat 



the endosperm results. To express the effect of such a "double dose of 



femaleness": 



let 'p = either a generative nucleus of a pollen grain which carries 

 the " Polish " factor or one of the polar cells which unite 

 to form the definitive nucleus of an ovule bearing the 

 "Polish" factor. Let k be similarly used for "Kubanka." 



From the gametogenesis of a heterozygous (/) plant the </ generative 

 nuclei will be one half {p) and one half {k) ; the % definitive nuclei will 

 be one half {2p) and one half (2A;). When such a plant is selfed the 

 following endosperms will result : 



3p, IpTk, 2k + p, Sk 

 Thus four types of endosperm are expected among the grains borne 

 by "/" plants. 



An endosperm of constitution Sp develops, when nourished by an 

 ^0 • P mother plant, into the characteristic " Polish " form of endosperm ; 

 but it may be that when nourished by the less vigorous "/" type of 

 plant it is imperfectly weaned. Consequently its final length may be 

 below the ^o • -P grain-length standard. Similarly Sk may be above the 

 Fq. K standard. The remaining types 2j9 + A; and ''Ik + p will presumably 

 lie between the ^o • P and ^o • K types and thus the expectation for the 

 grains borne by "/" plants is four constituents whose length-modes 

 have, owing to irregular development of the endosperms, closed inwards. 

 The intensified overlapping would certainly produce unimodality of the 

 whole distribution. 



This theoretical consideration casts no new light upon the problem. 

 As will be observed, it might be elaborated into an explanation of 

 " shift " — an embryo, genetically Polish, fed during development on an 

 "I" plant and nourished during germination by a similarly improperly 

 weaned (3p) endosperm, may be e'xpected to grow into a reduced or 

 "shifted" P plants Several experimental methods of investigating 



^ The probable validity of such an explanation of ' ' shift " is very greatly increased by 

 the interesting facts collected by Kidd & West [(18) especially Part iv, pp. 222 — 234]. AJclose 

 — almost linear — relationship is claimed between the amount of food reserve available for 

 the embryo and the adult vigour and ultimate yield of the plant resulting from the seed. 

 If this claim be justified, it is to be expected that in the P x K cross, there will be a 

 relationship between seed-weight and length of glume of plant resulting from the seed. 

 That Fj . K glume-length exceeds Fq. K is consistent with the explanation suggested, for 

 a 3k endosperm nourished by an F^."!" mother plant is likely, on an average, to be 

 heavier than if nourished by an Fq . K mother plant. In the same circumstances an 

 increase in vigour of the embryo accompanying the endosperm is perhaps a legitimate 

 expectation. 



