Reduction division 

 in spore mother cell 



Division of two 

 haploid nuclei 



Formation of four 

 haploid cells 



Four liberated 

 pollen grains 



THE ORIGIN OF POLLEN GRAINS 



In the pollen mother-cell the nucleus undergoes two divisions without the formation of 

 cell-walls. In one of the divisions the chromosomes are reduced to half the normal 

 number. Around each of the four nuclei a thickened cell-wall is formed. This more 

 or less rounded cell becomes separated from the others and is a pollen grain 



What the earUer gardeners and biologists did not know, and could not know 

 until certain microscopic studies had been made, is that in "fertilization" a 

 haploid nucleus from a pollen grain gets into the embryo sac and fuses with 

 the particular haploid nucleus which we have called the "egg", or female 

 gamete. 



The Anther as the Male Organ With the help of a microscope we can 

 distinguish inside an anther the cells that are to produce pollen grains (see 

 illustration above). These pollen mother-cells contain dense, granular pro- 

 toplasm. In each mother-cell the nucleus divides, and each new nucleus di- 

 vides again, but no cell-walls are formed. In either the first or the second 

 division, varying with the species, the number of chromosomes becomes 

 reduced to the haploid number (see page 386). The four haploid nuclei be- 

 come separated, and a thickened wall is formed around each, with its 

 cytoplasm. 



In the formation of these "male spores" the mother-cell gives rise to four 

 spores. In the formation of the embryo-sac nuclei, the original mother-cell 

 gives rise to only one female nucleus, the other three disappearing. However, 

 the protoplasmic material is not destroyed, but becomes organized around the 

 single female nucleus. 



We see, then, that in flowering plants the male and female gametophytes 

 are reduced to single cells. Yet inside these cells very complex activities take 

 place, leading to the formation of a single gamete in each case — the male and 

 the female. 



403 



