ANGIOSPERMS. 



391 



The pollen-tube having passed through the micropyle either encounters the 

 exposed apex of the embryo-sac, or, as in Watsonia and Santalum, the projecting 

 synergidae ; but very frequently a portion of the tissue of the apex of the nucellus 

 is still in the way, and the tube must penetrate it to reach the embryo-sac. The 

 wall of the latter is often weak and is sometimes indented by the advancing end 

 of the tube, or even, as in Canna, pierced by it. 



FIG. 321. Development of the embryo-sac, fertilisation and embryogeny in an orchidaceous plant, a combination 

 of figures of Gymnadenia. Conopsea and Orchis fallens. I rudiment of the mother-cell of the embryo-sac, and of the 

 inner integument. // division of this mother-cell of the embryo-sac into three sister-cells, the lowest of the three being the 

 rudimentary embryo-sac. /// displacement of the two upper sister-cells by the growing embryo-sac ; two nuclei in the 

 sac. /J' doubling of the two nuclei, ^further division of these .nuclei. VI the egg-apparatus in the anterior extremity 

 of the embryo-sac consisting of the two synergidae and the oosphere beneath them ; the three antipodal cells are in the 

 posterior end of the embryo-sac, and there are two free nuclei in its cavity. VII the mature ovule before fertilisation ; 

 the two free nuclei of the embryo-sac have coalesced and formed a secondary nucleus which still contains two nucleolL 



VIII fertilisation by means of one of the synergidae (the one to the right) which appears to be changed into a homo- 

 geneous highly refractive protoplasmic body; there are two nuclei, the sperm-nucleus and the nucleus of the oosphere. 



IX pro-embryo consisting of two cells. X further development of the pro-embryo. The letters em denote the mother- 

 cell of the embryo-sac (archesporium), e the embryo-sac, j>ek the primary nucleus of the embryo-sac, sek the secondary 

 nucleus of the same, s the synergidae, o the oosphere, g the antipodal cells, p the pollen-tube, k the pro-embryo in process 

 of differentiation, it the inner, ai the outer integument, f the funiculus, h an intercellular space. From drawings by 

 Prof. Strasburger. VII is magn. about 200 times, the rest about 300 times. 



The contact of the tube with the apex of the embryo-sac or with the filiform 

 apparatus of the oosphere is sufficient for the transference of the fertilising substance ; 



