I4I6 



A TEXTBOOK OF THEORETICAL BOTANY 



Fig. 1307. — Micropylar projection of embryo sacs. A, Popuhis tremuloides. B, Torenia 

 asiotica. C, Utricidaria vulgaris var. americana. Micropylar haustorium of embryo sac 

 invading nutritive tissue of placenta. (B after Engler. C after Wylie and Yociun.) 



sacs are embedded either in the central 

 placenta or in outgrowths from it which 

 may be regarded as naked nucelli. The 

 embryo sacs become immensely elongated, 

 grow out into the ovarian cavity, follow up 

 the surface of the placenta to its apex and 

 from there, in some genera, continue up into 

 the stylar canal, reaching even almost to the 

 stigmas. They also extend somewhat in the 

 chalazal direction, though this growth is 

 limited by the resistance of the tissues 

 (Fig. 1309). 



As in all these cases the synergidae and 

 the oosphere are carried forward by the 

 growth of the embryo sac, fertilization must 

 be promoted by an earlier meeting with the 

 pollen tubes. One may recall, in this con- 

 nection, the curious embryo sac tubes of 

 Welwitschia (see Volume I, p. 776) which 



grow up through the massive nucellus to meet the descending pollen tubes. 

 The endospermal haustoria and the antipodal haustoria or caeca of the 



embryo sac are mostly post-fertilization structures and will be dealt with 



later in their proper sequence. 



The Oosphere. The position of the oosphere is really lateral to the syner- 



FiG. 1308. — Utricularia stellaris. 

 Younger stage of the embryo 

 sac development shown in 

 Fig. 1307 C. {After Goebel.) 



