IMPROVEMENTS OF PLANTS 43 



these plants may be grown side by side without danger of 

 mixing. An open-fertilized flower is one that may be ferti- 

 lized by pollen from another flower either on the same plant 

 or on a different one. In many plants, it may be fertilized 

 either by its own pollen or by that from some other flower; 

 in some, it must be fertilized by pollen from another flower, 

 and the results are better if this flower is on another plant. 

 In corn, open pollination is assured, because the pistillate 

 and staminate flowers are on different parts of the plant, and 

 the pollen is not shed till several days after the ovules on that 

 stalk are ready for fertilization, so that corn growing in a 

 field is almost certain to be cross-fertilized. As corn pollen 

 is carried for some distance by the wind, two varieties of corn 

 planted near together are almost certain to mix if they 

 "silk" at about the same time. 



46. Crosses and Hybrids. A cross is produced by the 

 union of two varieties; if Reid's Yellow Dent corn is fertilized 

 with pollen of Learning or Boone County White or any other 

 variety, the result is a cross. In the original meaning of the 

 word, a hybrid was the product of the union of two species, 

 as wheat and rye, but the term is now commonly used in the 

 sense of a cross between varieties of the same species. True 

 hybrids are seldom fertile; that is, they will not produce 

 seed. Thus in the case just mentioned, numerous hybrids 

 of wheat and rye have been produced, but in very few cases 

 have these hybrids produced seed which would germinate. 

 The process of cross-fertilization by artificial means is fre- 

 quently used for the production of new varieties. Con- 

 siderable care, time, and selection must be devoted to crosses 

 in order to get them to "come true," i. e., to produce uni- 

 formly the type of plant which is desired. 



47. Improvement by Selection. The principle that like 

 produces like is used by plant breeders in the improvement 



