VIIl] 



FERTILISATION 



121 



When the pollen-grain, having adhered to the hairy 

 stigma, has begun to germinate, the resulting pollen-tube 

 creeps down between the cells 

 of the stigma, and hands over 

 its enclosed nuclei to the em- 

 bryo-sac, where fertilisation of 

 the egg-cell is accomplished, 

 by the fusion of one of the 

 pollen nuclei with the nucleus 

 of the egg-cell. As the result- 

 ing embryo developes, the sac 

 becomes filled with endosperm- 

 cells charged with starch- 

 grains or sugar, and in the 

 ripe seed the embryo is always 

 found affixed laterally and be- 

 low to this endosperm — a point 

 of distinction from Sedges, 

 where the embryo is buried 

 in the endosperm. 



The ripe seed fills the 

 ovary, and its outer walls 

 usually fuse with those of 

 the carpel, forming the well- 

 known Caryopsis or "grain." 



If such a " grain " is care- 

 fully examined, three chief 

 parts are visible in addition 

 to the embryo. (Fig. 38.) 

 Firstly, we find on the outside the fused seed- and fruit 



Fig. 38. Longitudinal median 

 section of the caryopsis of a 

 grass — e.g. Lolium ( x about 

 35). p, pericarp; t, attach- 

 ment to axis ; m, position of 

 micropyle; E, endosperm ; A, 

 its aleurone layer; I, folded 

 leaves of plumule; r, radicle; 

 r", secondary roots ; RG, root- 

 cap ; s, scutellum. The dark 

 line e represents the surface 

 where the face of the scu- 

 tellum is applied to the endo- 

 sperm and where absorption 

 of the latter takes place. 



