CAUSE OF HYBRID VIGOR 183 



Our hypothetical illustration of characters contrib- 

 uted by both parents may be supported by actual results 

 from a cross between inbred strains of maize. As men- 

 tioned before, maize strains have been obtained which lack 

 brace roots and are unable to stand upright when the 

 plants become heavy. These strains, however, have the 

 habit of branching freely from the base of the plant, thus 

 producing several main stalks from each seed. When this 

 strain is crossed with one which possesses weU-developed 

 roots, but which does not branch at the base of the plant, 

 the result is an extremely vigorous progeny which pro- 

 duces several stalks from each seed and which shows no 

 deficiency in root development. The hybrid plants are 

 so large and so exceedingly vigorous that other factors 

 must have been involved, but these two characters can 

 easily be seen to have contributed something to the 

 luxuriant development. 



An even more striking illustration was obtained by 

 Emerson. A dwarf race of maize which was almost com- 

 pletely sterile was crossed with a tall plant which was 

 so deficient in chlorophyll production that it was un- 

 able to produce seed, although it had some functional 

 pollen. The hybrid plants were tall, dark green and 

 produced well-developed ears. Here normal stature was 

 contributed by one parent and proper chlorophyll devel- 

 opment by the other, the progeny was thereby enabled to 

 develop well and to become highly productive. 



In both these crosses the characters involved are 

 largely of a superficial nature, although most of them are 

 necessary for full development. They are characters 

 which are easily seen and serve as an indication of what 

 must have occurred in the case of other factors more in- 



