274 



REPRODUCTION 



on the stigma of the carpel of a suitable flower it germinates 

 and produces a pollen-tube which penetrates into the tissues of 

 the stigma and grows down through the style into the cavity 

 of the ovary : the time taken to reach this point may vary from 

 a few hours to several weeks, according to the kind of plant. 



^ 



The advancing pollen-tube is 

 guided in some way not com- 

 pletely understood into the micro- 

 pyle of the ovule and at length 

 comes into contact with the apex of 

 the embryo-sac close to the egg- 

 apparatus (Fig. 99). On reaching 

 this point its tip becomes dis- 

 organised and one of the genera- 

 tive cells of the pollen-grain travels 

 on through the open end of the 

 tube until it meets the ovum or 

 oosphere. The generative cell and 

 ci_^ the ovum then fuse into one, their 

 parts becoming completely inter- 

 mingled. This fushion of a genera- 

 tive cell with the ovum is the essen- 

 tial feature of the sexual act and is 

 spoken oS. 2& fertilisation. 



In several instances the second 

 generative nucleus from the poUen- 



the arrangement of the various parts groin has rerpntiv hppn fniinH tn 

 at the time of fertilisation, o Ovary; 6^'^'" ""^^ recently Oeen lOUnU 10 



fuse similarly with the definitive 

 nucleus in the embryo-sac, and this 

 ' double fertilisation process ' is pro- 





Fig. gg. — Diagramof a longitudinal 

 sectioniof a carpel containing an ortho- 

 tropous ovule : designed to illustrate 



style ; st stigma of the carpel ; 

 p pollen-grain germinated on the 

 stigma ; pt pollen-tube ; r the genera- 

 tive nucleus ; / funicle ; ch chalaza ; 

 c integuments of ovule ; « nucellus ; 

 es embryo-sac ; e ovum or egg-cell ; 



^definitive nucleus; a antipodal cells, bably gcneralamongfloweringplants. 

 Unless the ovum is fertilised both it and the whole ovule wither 

 and die, but as Soon as fertilisation is effected the ovum com- 

 mences to divide and grow, developing into an embryo plant, 

 the whole ovule finally becoming a seed. 



