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A TEXTBOOK OF THEORETICAL BOTANY 



understood. It may, however, protect the cell from bursting at the upper 

 end during fertilization, as we shall see later. It may also have a secretory 

 function in attracting the pollen tube, since the tube generally enters 

 through it. It appears to be tough and durable as it often outlasts the dis- 

 integration of the rest of the cell. The lower, rounded end of each synergid 

 contains a large vacuole, which also plays a part in fertilization. 



In the Compositae and possibly in one or two related families, the syner- 

 gidae break through the embryo sac wall and extend upwards into the 

 micropyle. These plants being tenuinucellate, the nucellus has by this time 

 disappeared, so that there is direct access from the sac to the micropyle. 

 In one or two genera, such as Calendula, they even grow out of the micropyle 

 into the ovarian cavity (Fig. 1305). It is not always possible to be certain 



Fig. 1305. — Calendula lusitajiica. A, Mature embryo sac with projecting 

 synergidae. B, Later development of the micropylar haustorium with 

 remains of the synergidae at the upper end. {After Billings.) 



