CHAPTER IV 

 THE MEASUREMENT OF VARIATION 



IT is often desirable to describe a plant or a group of 

 plants in exact mathematical terms. Most of the plant 

 characters with which a breeder deals are measurable, 

 and an individual plant may be described as having so 

 many leaves, so many grains, and so on throughout a 

 long list of measurements ; or a group of plants may be 

 expressed in the form of averages ; likewise, the degree 

 of resemblance or difference between plants and their 

 offspring, or among plants of a certain group or " popula- 

 tion. " The degree or extent of correlation or association 

 of plant characters may also be expressed mathematically. 



The science of biometry. The expression of variation 

 and heredity by means of statistical methods is known as 

 the science of Biometry. This method of description is 

 now being widely employed by experimental plant-breeders. 

 It is another tool which the breeder uses to record his 

 progress and describe his plants. The biometrician 

 should be cautioned to keep his use of mathematical 

 treatment subservient to the biological facts, not forgetting 

 that biometry is simply a means toward an end and not 

 an end in itself. It is better first of all to become ac- 

 quainted with the real plants before any mathematical 

 treatment of their variability is attempted. It is often 



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