10 CONWAY ZIRKLE 



many supposed instances where the pollen influenced directly the color 

 and form of the flowers, the flavor and shape of the fruits, and the color 

 and content of the seeds. How many of these cases were really due to Men- 

 delian segregation we will probably never know, since the investigators did 

 not know enough to take proper precautions. 



We can, however, divide the history of true xenia into three periods: 

 first, when its visible effect was considered a lusus naturae (1588); second, 

 when it was known to be caused by foreign pollen (1716); and third, when 

 the embryo and endosperm were recognized as two different structures and 

 when the influence of the pollen upon the latter was recorded specifically. 

 In the paragraph on Zea in the section on xenia, Focke cites the work of 

 Vilmorin (1867), Hildebrand (1868), and Kornicke (1876), who described 

 the effect of pollen on the endosperm. 



We should note a brief comment on the subject which has been overlooked 

 and is earlier than the papers cited by Focke. In 1858, Asa Gray described 

 xenia in maize. He reported starchy grains in ears of sweet corn and many 

 different kinds and colors of grains on the same ear. He had two explana- 

 tions for this occurrence: (1) cross pollination of the previous year and (2) 

 direct action of the pollen on the ovules of the present year. It is obvious 

 that by ovules he did not mean embryos. This may be the earliest authentic 

 recognition of the real problem of xenia. 



In reviewing the nineteenth century records of hybrid vigor in Zea mays, 

 we start with those of Charles Darwin (1876). Darwin planned his experi- 

 ments most carefully. He crossed and selfed plants from the same stock, and 

 raised fifteen plants from each of the two types of seed he had obtained. 

 He planted the seed from both the selfed and crossed plants in the same 

 pots, from six to ten plants per pot. When the plants were between one and 

 two feet in height, he measured them and found that the average height of 

 the plants from the selfed seed was 17.57 inches, while that from the crossed 

 seed was 20.19 inches or a ratio of 81 to 100. When mature, the two lots 

 averaged 61.59 inches and 66.51 inches, respectively, a ratio of 93 to 100. 

 In another experiment when the corn was planted in the ground, the ratio 

 of the selfed to the crossed was 80 to 100. Darwin called in his cousin, Francis 

 Galton, to check his results and Galton judged them to be very good after 

 he had studied the curves that he drew. 



The direct connection between Darwin's work and our present hybrid 

 corn is shown by Darwin's influence on W^ J. Beal who was the real leader 

 in the American research designed to improve maize. Beal reviewed Dar- 

 win's book in 1878, and even wrote an article which was little more than a 

 paraphrase of what Darwin had published. Beal's own contributions ap- 

 peared a little later. 



In 1880, Beal described how he had increased the yield of corn on a large 

 scale. Two stocks of the same type of corn which had been grown a hundred 



