VARIATION 



349 



Although all characters of the wheat plant fluctuate between a higher 

 and lower limit, there are great differences in the extent and range of their 

 variability. Of the characters mentioned above, the number of spikelets 

 per ear, length of straw and ear, number of internodes per straw, and 

 looo-grain weight vary much less than the yield per plant, weight and 

 yield of grain per ear, and the " tillering " or number of straws produced 

 by each plant. 



As already stated, individual variations or fluctuations are not in- 

 herited, and are considerably influenced by soil and season ; nevertheless, 

 the mean and range of variation of the different characters are hereditary 

 features of the group or population constituting the pedigree cultures 

 raised from a single plant. For example, the grains from an ear 60 mm. 

 long do not produce plants with ears of this length only, but a fluctuating 

 series, having the same mean and standard deviation as those of the 

 pedigree culture from which the original grain was derived. 



CORRELATIONS. The different organs of a plant are so co-ordinated 

 with each other that variation in one part involves a simultaneous variation 

 in another. In wheat, for example, variation in the number of straws 

 or tillering is associated with an alteration in the total weight of the 

 plant, and length of growing period with the total yield of grain per 

 plant. 



Such correlated variation may be positive ( + ), an increase in one of the 

 characters carrying with it an increase in the other, or it may be negative 

 (-), in which case an increase of one is linked with a decrease in the 

 other. 



While in many instances the facts of correlation are undoubted, the 



