128 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



twice, to give rise to a group of 4 nuclei which form the egg ap- 

 paratus and the upper polar nucleus (Fig. S2E-G). Of the re- 

 maining 2 nuclei, one may be considered as an antipodal and the 

 other as the lower polar nucleus. 



Several variations in the development and organization of the 

 embryo sac were found, however. Most of these seemed to have 

 their origin in the behavior of the middle nucleus. In some cases 

 it was found to take part in the third or fourth division, resulting 

 in a 7-nucleate embryo sac with two antipodal nuclei instead of one. 

 Less frequently it divided synchronously with the micropylar nu- 

 cleus, but only one of its daughter nuclei divided again, resulting 

 in an 8-nucleate embryo sac. 



It is probable that at least some of the variations reported by 

 Stenar, Eysel, and Fagerlind are due to environmental influences, 

 and a more detailed study is necessary to decide the point. 110 



Balsamita vulgaris. A recent investigation of the embryo sac 

 of this plant (Fagerlind, 1939c) has revealed several interesting 

 features. As in other Compositae, the ovules are tenuinucellate. 

 The archesporium is usually two-celled (Fig. S3 A), but sometimes 

 three cells may be present and occasionally there is only one. After 

 the first meiotic division 2 nuclei are formed of which the upper soon 

 becomes larger than the lower (Fig. 831?). Both divide again with- 

 out wall formation and the resulting 4 nuclei take up a 1+3 ar- 

 rangement (Fig. 83C-D). Only the micropylar nucleus functions, 

 while the other 3 nuclei soon begin to degenerate. Vacuolation 

 takes place at this stage and is followed by the appearance of a 

 lateral vesicular outgrowth, which assumes a tubular form and 

 gradually makes its way upward into the micropyle (Fig. S3E-F). 

 The functioning megaspore nucleus, which has by this time moved 



lla Mason (1949), who has made a recent study of Limnanthes, regards the 

 embryo sac as bisporic. 



Fig. 83. Development of embryo sac in Balsamita vulgaris. A, l.s. nucellus 

 showing two-celled archesporium. B, mother cell on right has two nuclei (end of 

 Meiosis I); that on left has four nuclei (end of Meiosis II). C,D, megaspore nuclei 

 take up 1+3 position; micropylar nucleus has enlarged; smaller chalazal nuclei 

 are on way to degeneration. E, formation of vesicular outgrowth from chalazal 

 end of the cell. F, functional megaspore nucleus has entered vesicle. G-H, two- 

 and four-nucleate stages. /, eight-nucleate embryo sac; note three nonfunctioning 

 megaspore nuclei at base. J, mature embryo sac, showing egg apparatus, second- 

 ary nucleus, and multinucleate antipodal cells. (After Fagerlind, 1989c.) 



