Dec. 1, 1919 Pistillate Spikelet and Fertilization in Zea mays L. 263 



FERTILIZATION 



After the entrance of the pollen tube into the embryo sac, it expands 

 so that the width of its tip is approximately one-third that of the embryo 

 sac. The pollen tube extends into the embryo sac until the tip is near 

 the polar nuclei. The wall of the tube is dissolved, giving the nuclei free 

 access to the embryo sac. One of the sperm nuclei fuses with the egg and 

 another with one of the polar nuclei (Pi. 31). The two polarnuclei fuse 

 at the time the sperm nucleus enters one of them or shortly afterwards. 

 Traces of the pollen tube in the embryo sac remain for a long time and do 

 not disappear until crowded out by the developing endosperm and 

 embryo. Fertilization takes place in from 26 to 28 hours after pollina- 

 tion, or in a few hours after the pollen tube reaches the embryo sac. 



DEVELOPMENT OF THE EMBRYO AND ENDOSPERM 



Almost immediately after fertilization, the endosperm nucleus begins 

 to divide ; and in 10 to 12 hours the nuclei of the endosperm may number 

 20 or 30, arranged around the periphery of the embryo sac (Pi. 32, A). 

 Many of the nuclei have two nucleoli. The nucleus of the fertilized egg 

 does not divide very rapidly. When the nuclei of the endosperm number 

 as high as 20 or 30, the egg nucleus has just undergone its first division 

 (PI. 32, A). The cells of the endosperm increase very rapidly, and within 

 36 hours after fertilization they completely fill the embryo sac (Pi. 32, B). 

 The antipodals remain intact and increase in number but are soon 

 crowded out by the encroaching endosperm cells. By the time the 

 endosperm completely fills the embryo sac the embryo consists of only 

 from 14 to 16 cells (Pi. 32, C). 



SUMMARY 



In a study of the pistillate spikelet and the process of fertilization in 

 the corn plant (Zea mays) the following facts were noted : 



Embryo sac. — In the formation of the embryo sac there is no dis- 

 organization of the megaspores, and all four function. The three antip- 

 odal cells rapidly increase in number, apparently by indirect cell division, 

 until they number from 24 to 36 at the time the embryo sac is mature. 

 These cells have rather indistinct cell walls and frequently contain two 

 nuclei. The two polar nuclei come into position just above the egg and 

 remain in close contact with each other but never fuse before fertilization 

 has taken place. The egg becomes reticulate, stains very lightly, and 

 is decidedly balloon-shaped. 



Pollen tube. — Practically all the pollen tubes that function come 

 from the pollen grains that lodge on the hairs of the silk. The tubes may 

 enter the hairs directly and through them gain access to the interior of 

 the silk, or they may follow the hairs to their base and then penetrate 

 the silk. After the pollen tubes are once inside the silk they work their 



