168 D- F. JONES. 



The greatest difference in fertilizing efficiency was found when 

 a small seeded variety of popcorn of the Zca mays cverta type 

 having very corneous and pointed seeds was used. The selective 

 action was so pronounced that it seemed worth while to repeat the 

 tests with this type on a more extensive scale. At the same time, 

 the pollinations were made and the data recorded in such a way as 

 to give an indication as to whether or not the result was due to a 

 differential rate of pollen tube growth. 



The seeds of maize are arranged regularly on a central spike. 

 Each ovule has a separate pistil and these form a mass of fine fila- 

 ments which extend beyond the enclosing leaf sheaths. See Fig. 

 i. Just before the pollinations were made the pistils were cut 

 off evenly at a short distance beyond the tip of the spike, and the 

 pollen mixture was applied to the cut ends of the filaments. The 

 distance that the pollen tubes had to travel to reach the ovules 

 differed considerably in the case of the seeds produced in the tip 

 of the spike as compared with those at the base. The mature 

 pistillate inflorescences of the material worked with ranged from 

 ten to twenty centimeters in length. At the time fertilization took 

 place they were considerably shorter than this. It is estimated 

 that the pollen tubes travelled through a distance which varied 

 from about five to fifteen centimeters. If the plant's own pollen 

 tubes grow faster than the foreign tubes, we would expect fewer 

 cross-fertilized seeds at the base of the spike than at the tip. 



Five different mixtures of pollen were made and applied to 

 about ten plants of each of the two contrasted types. For the 

 white, smooth-seeded type a first generation hybrid of two inbred 

 strains of a variety of sharply pointed popcorn known as Squirrel 

 Tooth was used, and for the yellow, wrinkled type another .first 

 generation hybrid of two inbred strains from a variety of sweet- 

 corn known as Golden Bantam. Hybrid plants were employed 

 because of their size and vigor, making it possible to secure a 

 large number of seeds from a single application of pollen. The 

 plants of each type were characteristically uniform, and were prac- 

 tically identical in genetical constitution. They were producing 

 segregating gametes but these were presumably alike in respect 

 to their cytoplasmic covering. In any case the gametic differences 

 were no greater than are present in the original varieties. The 



