EMBRYO SAC 



367 



resulting structure of the embryo sac are practically identical in 

 the vast majority of the Angiosperms (both Monocotyledons and 

 Dicotyledons) which have been investigated, and this uniformity 

 constitutes one of the strongest arguments for the origin of the 

 group from a common ancestry. Tubular sucker-like outgrowths 



FIG. 215. Germination of pollen and fertilisation. A-D, Lily (Lilium) 

 (after Guignard). E-F, Tulip (Tulipa) (after Ernst). A, Mature 

 pollen grain, showing the single nucleus. B and E, Later stage, showing 

 generative cell (g.c.) and tube nucleus (t.n.). C, Tip of pollen tube 

 with tube nucleus (t.n.) and two male cells ($). D, Embryo sac at 

 the moment of fertilisation, showing antipodals (a.), polar nuclei (p.n.), 

 ovum (o.), one synergid (s.), and the two vermiform male cells ($) 

 discharged from the tip of the pollen tube (p.t.}. F, Early stage in 

 formation of pollen tube. 



from the chalazal or both ends of the embryo sac are met with in 

 the Beech, Hornbeam, and some Sympetalae (Plantago lanceolata, 

 Scrophulariaceae) , and probably aid in the transference of food- 

 material to the developing embryo. The same end is served 

 by the haustorial outgrowths from the later-arising endosperm 

 (see below) which occur in some genera. 



Having passed through the micropyle, the pollen tube pene- 



