414 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



ez ek d bk 



hz 



o bk 



Fig. 214. Diagrams illustrating derivation of angiosperm embryo sac from that 

 of coniferous ancestor (bk = ventral canal nucleus ; d = archegonial jacket ;ek = egg 

 nucleus; ez = egg cell; hz = neck cells; i = nonfunctional archegonium; p = pro- 

 thallus; ps = pollen tube; sta = sterilized archegonia forming archegonia jacket). 

 A, female gametophyte of hypothetical gymnosperm, essentially similar to modern 

 Sequoia. There are several laterally situated archegonia, each with its own jacket. 

 As a rule each pollen tube fertilizes a single archegonium, but rarely one tube may 

 fertilize two archegonia (left bottom). B, female gametophyte of Cupressus type, 

 showing fewer archegonia arranged in a compact group and enclosed in common 

 jacket. C, female gametophyte of Ephedra, showing further reduction in number 

 of archegonia. Jacket is supposed to have arisen by the sterilization of some 

 archegonium initials. D, hypothetical transitional stage, showing reduction in 

 size of gametophyte and disappearance of arohegonial jacket. E, gametophyte 



