THE DEVELOPMENT OF SEXUALITY IN PLANTS 



177 



as leaves which have become metamorphosed from their vegetative 

 form and function to hold the sex structures. The stamens and 

 pistil (carpel) contain spore-forming tissues which, by means of 

 reduction division, produce pollen grains containing microspores 

 (sperms), while ovules produce a female gametophyte and its egg. 

 The sperm cells are formed in the pollen grains, while the egg cells 



germinating poller 



tube 



osUs of 

 anLher 



form pollen grains with— ^ sperm 2,-^ ^; 



tuba nuclau^ 



TRe embr/o sac contains a dividimg nuclaus ^ v '/ bolUn 

 eight cxne jlnally formed, tuJo forn-i fusion rnAcleu^ /^tL-ubo 



Development of male and female gametophyte in the flowering plants. Only 

 the cells which actually form these structure are shown. The parts of the sporo- 

 phyte upon which the gametophyte is parasitic are omitted for the sake of clarity. 

 Read the text carefully and then use the diagrams. 



are held within the ovary of the pistil as has been previously stated. 

 In the angiosperms or flowering plants the male gametophyte is so 

 much reduced that it consists of only three cells, a tube nucleus and 

 two generative cells (see figure). Just previous to the formation of 

 the pollen grains (male gametophyte) reduction di\ision takes place 

 so that its cells contain the haploid number of chromosomes. The 

 female gametophyte is also greatly reduced. After reduction divi- 

 sion, the megaspore divides (see figure) one nucleus migrating to each 

 end of the etnbryo sac (female gametophyte). The nuclei continue 

 to divide until eight are formed in two groups at opposite ends of 

 the embryo sac. From each group a single nucleus then unites with 

 the other to form a fusion nucleus (see figure). At this stage the egg 

 nucleus is ready for fertilization by the sperm nucleus. A double ferti- 

 lization now takes place, the sperm nucleus fuses with the egg nucleus 

 and the second sperm nucleus unites with the fusion nucleus. The 



