3IO COLUNS: INTOLERANCE TO SELE-FERTILIZATION 



require special care to keep them alive after four or five successive 

 self-pollinations. In view of this serious limitation it seems re- 

 markable that the species has developed no adequate means of 

 avoiding self-pollination. 



The staminate and pistillate flowers of maize are borne on 

 different parts of the plant, but most varieties are synacmic, or 

 at most slightly proterandrous, and as the staminate flowers are 

 at the top of the plant it is only when winds continue during all 

 the time pollen is being shed that self-pollination is avoided. 

 It is obviously of advantage to a plant with the sexes disposed 

 as in maize not to be entirely dependent on cross-pollination. 

 But it appears almost equally obvious that a slight departure 

 from synacmy toward proterogyny would be more advantageous 

 than a similar departure in the direction of proterandry. 



Maize may be successfully pollinated at any time within 5 

 to 10 days after the emergence of the silks. With a variety 

 normally proterandrous, pollen continues to fall from a plant for 

 one or two days after the silks emerge. If a period of calm pre- 

 vails at this time, the ear will be fertilized by pollen from the 

 same plant. Should there be wind during the forenoon, when 

 most of the pollen is liberated, or if the plant be slightly more 

 proterandrous, fertilization will be dependent on pollen from 

 other plants, and if no foreign pollen is at hand the ear will be 

 sterile. If, on the other hand, the silks were to appear before 

 the pollen, there would be the same opportunity for cross-polli- 

 nation as with proterandrous plants, and should no foreign 

 pollen be available, pollen from the same plant beginning to 

 fall while the silks were still receptive would give self -pollinated 

 seed instead of a sterile ear. 



From the behavior of varieties imported from the tropics it 

 was at one time thought that the more primitive varieties of 

 maize were more proterandrous, and that the practically synac- 

 mic nature of improved varieties was the result of intensive 

 breeding.^ As selection is usually practiced, markedly pro- 

 terandrous individuals would be considered barren stalks and 



^ Collins, G. N. A variety of maizt with silks maturing before the tassels. 'U.S. 

 Dept. Agric. Bur. PI. Ind. Circ. 107. February 7, 1913. 



