260 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



it. To quote his words, "It is not improbable that in some cases the 

 second sperm nucleus enters the embryo sac, but fails to unite with 

 the two polar nuclei. In such cases it may be able to form a spindle 

 and divide separately, the unfecundated embryo sac nucleus formed 

 by the union of the two polar nuclei also dividing separately. If 

 this occurs, there would then be formed in the protoplasm of the 

 embryo sac, nuclei of two distinct characters, one group from the 

 division of the embryo -sac nucleus and the other from the division 

 of the sperm nucleus." Since the nuclei become interspersed during 

 the free nuclear stage of the endosperm, such a hypothesis would 

 account for the occurrence of variegated kernels. Yet another 

 possibility which suggested itself to Webber was that the second 

 sperm nucleus may fuse with only one of the two polar nuclei and 

 that "after their fusion takes place the other nucleus is repelled and 

 develops independently." As in the preceding case, there would 

 thus arise two groups of nuclei — one from a fertilized polar nucleus 

 containing both maternal and paternal elements, the other from an 

 unfertilized polar nucleus containing only the maternal elements. 



None of Webber's postulates has found adequate microscopic 

 support up to this time. It is pertinent, however, to call attention 

 to three reports which bear upon this subject. 



In a study of Petunia, Ferguson (1927) claimed that endosperm 

 formation is initiated independently of fertilization and that the 

 pollen tube discharges its contents into the embryo sac only after 

 the two- or four-celled stage of the endosperm. One sperm nucleus 

 then fuses with the egg and the other with the nucleus of the upper- 

 most of the endosperm cells. Consequently the endosperm tissue 

 derived from this cell is triploid and the rest is diploid. She claimed 

 to have counted 21 chromosomes in the micropylar cells of the endo- 

 sperm and 14 in the chalazal cells. 



Bhaduri (1933) reported a similar condition in a strain of Lyco- 

 persicum esculentum. Here also the polar fusion nucleus is said to 

 divide before fertilization. Two cells result, a small micropylar 

 and a large chalazal. After the discharge of the pollen tube one 

 sperm fertilizes the egg and the other fuses with the nucleus of the 

 micropylar endosperm cell. 



The third report concerns Acorus calamus (Buell, 1938), in which 

 the pollen tube is said to enter the embryo sac only after the second- 

 ary nucleus has divided to form two chambers, a large micropylar 

 and a small chalazal, "Syngamy takes place normally but there is 



