168 COMPOUND ORGANS OF PLANTS. 



' 



- : ' , 



Fig. 71. Pistil of Asclepias a, with pollen masses p, adhering to the stigma s. 

 b. Separate pollen masses united by a gland like body. 



reception. We have seen that the nucleus is covered by two 

 membranes, called the primine and secundine, and that at the 

 "apex of the nucleus both coverings leave an opening which 

 has been termed the foramen or micropyle." Now this open- 

 ing, or the nucleus projecting beyond it, is the ultimate desti- 

 nation of the pollen tube. Before its arrival, however, one of 

 the cells towards the summit of the nucleus expands and thus 

 creates a cavity in its interior which is called the embryo sac, 

 because it is in the interior of this sac that the embryonal 

 vesicle first makes its appearance in the upper part of the 

 cavity. It is at first a simple cell which insensibly elongates, 

 and by the formation of transverse septa forms itself into a sort 

 of confervoid tube. The terminating cell of this tube enlarges 

 and forms the embryonal vesicle. The pollen tube, having 

 arrived at the base of the style, enters the ovary, and makes 

 its way through the micropyje or orifice of the ovule, pene- 

 trating the tissue of the nucleus till it reaches the embryo sac. 

 Fecundation appears to be produced by the simple contact of 



