152 PLANTS AND MAN 



perpetuating seedless fruits, double flowers, and variegated leaves. 

 It has been especially successful with wheat, oats, peas, beans, 

 tobacco and potatoes. 



Nature sometimes produces plants which have more con- 

 spicuous differences from others of the same species than is found 

 in the usual variations. Such a marked heritable variation is 

 known as a mutation; and the plant producing it is a mutant, or 

 sport. Many of our cultivated plants originated in such natural 

 mutants, especially among the apples. Early man discovered 

 these sports, transplanted them to his gardens and used them as 

 the beginning of a new line of descent. In most cases, perpetua- 

 tion of the character is brought about by asexual reproduction 

 involving the use of cuttings, bulbs or tubers, or grafting. 



Within more recent times, man has not waited for nature to 

 produce variations and mutations, but has discovered how to 

 bring them about himself. One of the most frequent causes of 

 variation is hybridization; that is, the crossing of plants with 

 differing characteristics but belonging, in most cases, to the 

 same species. As has been discussed in a previous chapter (see 

 p. 72) a seed contains a plant embryo which owes its existence 

 to sexual reproduction; half of its traits have been passed on 

 from a "male" parent through the pollen grain, the other half 

 from the "female" parent producing the ovule. In cross-polli- 

 nated plants, therefore, the seed (which is produced on the 

 pistillate or "female" plant) contains some traits which bear no 

 relation to the plant on which it is found. Hence no one can judge 

 what the characteristics of the new plant will be by simply 

 knowing what the seed-bearing plant was like. In nature, the 

 pollinating parent is usually unknown and therefore the seed 

 often germinates into a plant with unforeseen combinations of 

 traits. 



Scientific and controlled hybridization means knowing what 

 the traits of both parents of the seed may be. As far back as 1790, 

 a German botanist named Roelreuter took the pollen from one 

 kind of tobacco plant and placed it upon the pistil of another 

 variety of tobacco. This laid the foundation for the study of 

 hybridization in breeding plants. And in the following century 

 hybridization was used to produce as many new variations as 



