FERTILIZATION 49 



liantly colored corollas, have fragrant odors, and secrete nectar, a sweet 

 liquid very attractive to insects which are adapted to this work through 

 the possession of a pollen-carrying apparatus. Ex. : Orchids. 



Plants pollinated through the agency of water currents 

 are known as Hydrophilous. To this class belong such plants as 

 live under water and which produce flowers at or near the surface of 

 the same. Ex. : Sparganium. 



Some plants as the Honeysuckle and Nasturtium are fertilized by 

 humming birds. 



Before the pollen grain has been deposited upon the stigma a series 

 of events affecting both the pollen grain and the embryo sac occur. 

 The microspore (pollen grain) divides into two cells, the mother and 

 tube cells of the male gametophyte. The nucleus of the mother 

 cell divides to form two generative nuclei. The nucleus of the mega- 

 spore or embryo sac undergoes division until eight daughter nuclei 

 are produced which are separated into the following groups : 



(a) Three of these nuclei occupy a position at the apex, the lower 

 nucleus of the group being the egg or ovum, the other two nuclei being 

 the SYNERGiDS or assisting nuclei. 



(b) At the opposite end of the sac are three nuclei known as the 

 antipodals which apparently have no special function. 



(c) The two remaining nuclei (polar nuclei) form a group lying 

 near the centre of the embryo sac which unite to form a single nucleus 

 from which, after fertilization, the endosperm of nourishing material 

 is derived. This stage of the embryo sac constitutes the female 

 gametophyte. 



Fertilization. — After the pollen grain reaches the stigma the viscid 

 moisture of the stigma excites the outgrowth of the male gametophyte 

 which bursts through the coats of the pollen grain forming a pollen 

 tube. The pollen tube carrying within its walls two generative and 

 one tube nucleus penetrates through the loose cells of the style until it 

 reaches the micropyle of the ovule, then piercing the nucellus it enters 

 the embryo sac. The tip of the tube breaks and one of the generative 

 nuclei unites with the egg to form the oospore. The oospore develops 

 at once into an embryo or plantlet, which lies passive until the seed 

 undergoes germination. The other generative nucleus unites with the 

 previously fused polar nuclei to form the endosperm nucleus which soon 

 undergoes rapid division into a large number of nuclei scattered about 

 through the protoplasm of the embryo sac. These accumulate proto- 



