142' Elementary Principles of Agriculture 



six generations. At the end of this time the average of 

 the variety was twenty rows, whereas it had originally 

 been only thirteen. The lowest number of rows on any 

 ear was twelve and the highest twenty-eight, a number 

 that had never been observed in the parent variety. 

 The average and the actual number of rows had been 

 greatly increased by continuous selection through six 

 years; yet, when left for three years without selec- 

 tion, the average number of rows was back to thirteen. 

 Other instances might be mentioned, showing the in- 

 constancy of varieties propagated from seed. 



208. Perpetuating Desirable Variations. How may a 

 desirable variation be perpetuated? There are two ways: 

 (a) Propagating the Plant by Division, (b) By Repeated 

 Selection toward an Ideal Type. Many kinds of plants are 

 more conveniently propagated from seed, such as the 

 grains, cotton, garden vegetables, and the like. We have 

 seen how the number of rows of grains on an ear of corn 

 was increased. Had the selections been continued for 

 ten or more years, the new characters would have been 

 more fixed. 



(c) Special Methods. In addition to continual selec- 

 tion, plant-breeders sometimes resort to inbreeding to 

 fix variations. Plants that normally inbreed, like oats, 

 wheat, cotton, and others, are much less variable than 

 kinds that are normally cross-fertilized, as corn. 



209. How to Stimulate Variation. While seed-propa- 

 gated plants are variable, in fact too much so for the 

 average grower, the plant-breeder desires to bring about 

 the most decided variations possible in the hope that 

 some form of unusual value may be secured. The means 

 usually relied upon are: 



(a) Intensive Culture. Plants grown under the most 



