216 AGRICULTURE. 



(3) Sexual Reproduction is probably the 

 most important of the causes of variation in 

 plants. This is exemplified by those lower forms 

 which usually reproduce asexually, since they 

 show very little variation in many generations. 

 On the contrary, among higher plants, where 

 reproduction is ordinarily sexual, the offspring 

 may vary greatly in one generation. (Thus, a 

 field planted in white or yellow corn may pro- 

 duce many variously colored ears.) Hundreds 

 of examples of both wild and cultivated plants 

 may be given. * 



This variation is due, in a great measure, to 

 the fact that each organism is the product of 

 two separate elements the male and the female. 

 And each of these was itself a product of two 

 separate elements. In this way the whole an- 

 cestral line was developed. These characteristic 

 differences are the more marked when the male 

 and the female elements are derived from differ- 

 ent individuals; for in the offspring is made 

 possible any of the characteristics, not only of 

 the immediate parents, but of the entire ances- 

 try of each. Thus, cross-fertilization becomes a 

 potent factor in producing variation. 



2. Fixation of Variation. — When variations 

 are beneficial to the plant in its present environ- 



* " No case is on record of a variable organism ceasing to vary 

 under cultivation. Our oldest cultivated plants, such as wheat, 

 still yield new varieties." — Origin of Species, Darwin, p. 6. 



