566 



BOTANY OF THE LIVING PLANT 



Telophase. 

 At the eighth and final stage of the first division {first telophase, Fig. 432, 7) 

 two daughter nuclei are formed and pass into a short resting stage {interphase). 



Homotype Division. 



A second division follows quickly in which the centromeres now split ; as a 

 consequence the process closely resembles an ordinary mitosis, the chromo- 

 some-halves passing to opposite poles and thus into different tetraspores 

 (Fig. 432, 8-10). 



The outstanding points of meiosis are : 



(1) It involves two nuclear divisions with only one division of the chromo- 

 somes. Hence, 



(2) It leads to a halving of the chromosome number (reduction). 



(3) Interchange of parts (crossing-over) occurs between the chromatids 

 of each pair of homologous chromosomes. 



(4) The first division of meiosis distributes the four chromatids of each 

 pair of homologous chromosomes, so that each daughter-cell receives two. 



(5) The second division distributes these two chromatids so that each 

 granddaughter-cell (tetraspores) receives one. 



(6) The four chromatids of each chromosome pair are thus distributed at 

 random among the four members of a tetrad of tetraspores. 







8 



■■■■ 



tfS 





\ 



10 



Fig. 432. 

 Meiosis — later stages in Pinus Banksiana. 1st Division : 6, anaphase ; 7, telo- 

 phase. 2nd Division : 8, metaphase ; 9, telophase ; 10, four pollen nuclei. (From 

 Sansome, after J. M. Beal, by courtesy of Scientific Horticulture.) 



Provided that each type of tetraspore is viable, the gametophytes and 

 ultimately the gametes formed by them will exhibit the same distribution of 

 genes as the spores from which they are derived. The whole gametophyte 

 generation and the gametes themselves will have the reduced or haploid 

 number of chromosomes. 



