8 CONWAY ZIRKLE 



especially struck by the vegetative luxuriance, root development, height, 

 number of flowers and hardiness of many of his hybrids. 



Naudin (1865) found hybrid vigor in twenty-four species crosses out of the 

 thirty-five which he made within eleven genera. In Datura his results were 

 spectacular. In reciprocal crosses between D. Stramonium and D. Tatula 

 the offspring were twice the height of the parents. Knowledge of plant 

 hybridization was increasing more rapidly at this time than the biologists 

 knew, for this was the year in which Mendel's (1865) paper Versuche iiber 

 Pflanzen-Hyhriden appeared. Mendel discovered hybrid vigor in his pea 

 hybrids and described it as follows: 



The longer of the two parental stems is usually exceeded by the hybrid, a fact which is 

 possibly only attributable to the greater luxuriance which appears in all parts of the 

 plants when stems of very different lengths are crossed. Thus, for instance, in repeated 

 experiments, stems of 1 ft. and 6 ft. in length yielded without exception hybrids which 

 varied in length between 6 ft. and 1\ ft. 



We shall cite but one more scientist who wrote on the general subject of 

 hybrid vigor in plants. This is Charles Darwin, whose Cross and Self Fertiliza- 

 tion in the Vegetable Kingdom appeared in 1876. This was a book of great 

 importance and influence, but no attempt will be made here to summarize 

 this work of nearly five hundred pages. At the beginning of his concluding 

 chapter, Darwin stated: 



The first and most important conclusion which may be drawn from the observations 

 given in this volume, is that cross-fertilization is generally beneficial and self-fertihzation 

 injurious. 



There is a special reason why this book of Darwin's is of such great 

 importance for any historical background to heterosis. Darwin worked 

 carefully and quantitatively with many genera, including Zea mays. He 

 measured accurately the amount of hybrid vigor he could induce, and he pub- 

 lished his data in full. His work stands in the direct ancestral line to the 

 twentieth century research on the subject, and the great advances made 

 from 1908 to 1919 are based solidly on this work. There are no great gaps 

 in the steady progress and no gaps in the literature. 



Zea mays was brought to Europe in 1493 by Columbus on his home- 

 ward voyage. This was sometime before the great herbals were written, 

 so our first descriptions of the new grain are to be found in the books of the 

 travelers and explorers. Later, Indian corn appeared under various names 

 in the early herbals, and it was described in detail in the famous Krautehuch 

 of Tabernaemontanus, first published in 1588. The author obviously yielded 

 to his enthusiasm in devoting five and a half folio pages to corn and includ- 

 ing thirteen illustrations in his treatment. He was the first to describe the 

 results of xenia — the occurrence of difi'erent colored grains on the same ear — 

 but his explanation of the phenomenon has nothing to do with cross pollina- 

 tion. He ascribed it directly to God Almighty. 



