ZEA MAYS 137 



The fine, light pollen is well adapted to wind pollination. 

 Gravity may play some part in the process, but it is unlikely that 

 many insects act as pollinating agents. A few hours after the pol- 

 len grain lodges on the hairs of the silk, the pollen tube pushes 

 through the germ pore and gains access to the sheath cells adjacent 

 to the vascular bundles of the silk. Practically all functional 

 pollen tubes develop from pollen grains that are lodged on the 

 multicellular hairs of the silk, although occasionally penetration 

 may be other than through a hair. Growth of the pollen tube in 

 the silk is very rapid and it may reach the embryo sac within twenty- 

 four hours after pollination. Upon the entrance of the pollen into 

 the embryo sac, fertilization is effected. This is double in charac- 

 ter, one microgamete uniting with the megagamete to form the 

 zygote, and the other with the polar nuclei to form the primary 

 endosperm nucleus. The latter undergoes division directly after 

 fusion; and, as a result of several free nuclear divisions, there are 

 usually four to eight endosperm nuclei formed before the division 

 of the zygote takes place. 



Embryogeny. — The zygote divides into two unequal cells about 

 i8 to 34 hours after pollination; and further divisions of the smaller 

 apical cell form the embryo, while the basal cell divides to form a 

 massive suspensor. The subsequent development has been de- 

 scribed in detail by Randolph (15). He observed three- or four- 

 celled embryos 36 hours after pollination; and at the end of four 

 days, they usually were comprised of from 10 to 14 cells. Unlike 

 the ontogeny of many grass embryos, there is little regularity in 

 the sequence of cell divisions or arrangement of the cells in tiers, 

 so that the proembryo cannot be described as conforming to a regu- 

 lar pattern. 



From the fourth to eighth days, growth is restricted chiefly 

 to the apical region, there being few divisions of the suspensor; 

 and, as a result of this differential growth, the embryo becomes 

 club-shaped. About eight days after pollination, the epidermis is 

 differentiated in the apical portion of the embryo and extends to- 

 ward the suspensor. 



The axis of the more mature embryo can be determined in about 

 ten days, when a slight protuberance is formed on the anterior sur- 

 face of the proembryo. This represents the apex of the stem axis 

 of the embryo and is oriented at an oblique angle to the pro- 

 embryo. During the period from ten to fourteen days after pollina- 



