CHAPTER XXIV 



VARIATION 



ONE of the most obvious peculiarities of living things is their variability. 

 No two human beings are alike in all their features, and no two plants 

 of the many thousands present in a field of wheat are exactly alike in all 

 their characters, even where the entire crop has descended from a single 

 ancestral grain. 



The form, size, and colour of the glumes, the height, strength, and 

 diameter of the straw, the length, weight, and number of spikclets of the 

 ear, and all other characters of the plant, morphological and physiological, 

 are subject to variation. 



Although variations may be classified and investigated from many 

 points of view, they fall naturally into two distinct groups, namely : 



(1) Fluctuations, or racial variations, and 



(2) Discontinuous variations or sports. 



FLUCTUATIONS 



Fluctuations pass by regular gradation from a lower to an upper 

 limit, and are distributed at random among the individuals forming u 

 crop. When measured, weighed, or counted, the results can be plotted 

 on frequency curves, which conform to the laws of chance. 



Variations of this class are produced by changes in the environment 

 of the plant ; they belong to the individual and arc not inherited. 



Examples of fluctuating variability are the lengths and numbers of the 

 spikelets of the ears of a crop of wheat. 



Of 500 ears taken at random from a field of " Swan " wheat, the 

 length varied between 50 and no mm. Measured and arranged in 

 groups differing from each other by 5 mm. the frequencies or numbers 

 of individuals in each class were 



LKN<;THS OK THK EARS (mm.) 



Class ' 5 55 ()0 f>5 ~ ~ 5 * 8 5 ( > ( J5 I0 I0 5 



1 54 59 6 4 <*) 74 79 84 89 94 99 104 109 



Frequency . i 9 28 30 93 94 98 77 49 10 <> 2 



Percentage in 

 each class . -2 i-S 5-6 6-0 18-6 18-8 19-6 15-4 9-8 2-0 r8 -4 



J-i: 



