Plants in Relation to Man 497 



As soon as the ovules are fertilized and the fruit set, the bag 

 should be removed. The stigma, which frequently withers 

 Jike corn silk, cannot then be affected by any pollen that may 

 fall upon it. The seed produced by controlled cross-fertiliza- 

 tion is kept separate from other seed, and is planted in isolation 

 from other plants. The progeny also must be hand-pollinated, 

 and their seed selected; and this process continued through 

 several generations until the desired variety has become rela- 

 tively permanent, reproducing true to kind. 



Limits of cross-pollination. Cross-pollination is usually 

 practiced with : (1) selected plants of the same variety ; (2) se- 

 lected plants of different varieties of the same species ; (3) se- 

 lected plants of different, but related species. 



Improving a variety. The first kind of cr'oss-tpollination is 

 usually employed in maintaining and improving the standard 

 of any given variety. . In a plot of peas of the same variety, 

 certain plants are selected because of their desirable characters 

 and cross-pollenized. From the seed of these a selection of 

 the best is made for the next planting. The plants grown 

 from such cross-fertilized seed combine in various ways the 

 desirable characters and are therefore generally superior to 

 the parent plants. From the offspring, selection is made 

 and cross-pollination effected, insuring improved or new 

 varieties. 



Related varieties. The pollen may be transferred from a 

 plant of one variety to the stigmas of a plant of another variety 

 of the same kind or species. One may cross-pollinate tall peas 

 with dwarf peas, wrinkled peas with smooth round peas. This 

 is a specially interesting field for experiment, the one in which 

 Mendel (pages 505 ff.) made his famous discoveries in inheritance. 

 The method is known as hybridization, or producing hybrids, 

 from which selection is made for propagation. Such hybrids 

 reproduce with remarkable certainty. 



Related species. A third possible combination is to transfer 

 the pollen from a plant of one species to the stigmas of a plant of 



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