60 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



ments arise near its apex (Fig. 43£), and (2) those in which the nucel- 

 lus is elongated and the integuments arise near its base (Fig. 43 A). 

 The Asclepiadaceae, Orobanchaceae, and Rubiaceae are good ex- 

 amples of the first condition, and the Orchidaceae of the second. 

 As the embryo sac matures, the nucellar cells gradually become 



C 



Fig. 41. L.s. ovules of Quisqualis indica showing progressively increasing amount 

 of parietal tissue, arising partly by divisions of the wall layers and partly by divi- 

 sions of cells of nucellar epidermis. A, B, megaspore mother cell stage. C, func- 

 tioning megaspore stage. In B and C, note enlarging cells of obturator. (After 

 Fagerlind, 19/, J.) 



