434 TEXTBOOK OF BOTANY 



proof of sex in plants." The experiments of several other investigators 

 during the 18th century were necessary to show that sex occurs in 

 all seed plants and that pollen is essential to the sexual process in 

 them. 



The actual process of fertilization as the union of a sperm cell with an 

 egg cell was unknown prior to the middle of the 19th centurv, and was 

 first discovered in the algae. The union of the egg in the embrvo sac with 

 a sperm from the pollen tube was first noted in 1884. The fact that the 

 endosperm of seeds develops from the triple-fusion nucleus formed by 

 the union of one sperm from the pollen tube with the fusion nucleus of 

 the embryo sac was discovered in 1898. The first careful experimental 

 study of the hybridization of plants is accredited to Koelreuter, 1761-66. 



Nevertheless, a considerable amount of controlled plant breeding was 

 accomplished by the method of trial and error previous to the 20th 

 century. Many new varieties of cultivated plants were obtained by 

 crossincr individuals of domesticated varieties with each other and 



o 



with wild species. Some new varieties were also obtained by crossing 

 wild plants with each other. The Concord grape, for instance, was ob- 

 tained by Ephraim Bull in Massachusetts by crossing two wild species. 

 This variety, which is now cultivated in most temperate regions of the 

 earth, was selected in 1853 from among 22,000 progeny of the two wild 

 parents. 



Toward the close of the 19th century new varieties of domesticated 

 plants were being obtained by foreign introductions, by cross-pollination 

 and inbreeding, by mass selection, and by single-line selection. 



As already indicated, a crop of cultivated plants is likely to be a 

 mixed population of several different varieties. In mass selection, seeds 

 from the more desirable plants in the field are collected and then planted 

 the following season. By this method the more undesirable varieties in 

 a mixed population are supposed to be eliminated and the more desirable 

 ones perpetuated. In a later chapter we shall see that the appearance of 

 an individual in the field may be verv deceptive with respect to the 

 kind of heredity it transmits to its progeny. In spite of this weakness of 

 mass selection it has been very effective in increasing the average yield 

 of many crops and is still considered a valuable practice. 



The method of single-line selection has recently become preferable to 



^ Camerarius was fortunate in choosing for experimental study plants in which par- 

 thenocarpy occurred, but in which parthenogenesis did not occur. Had he chosen dandelion 

 to study, his conclusions would have been different. 



