3° 



A TEXTBOOK OF THEORETICAL BOTANY 



chromosome number is diploid (2//). It follows therefore that, as the 

 chromosome number of a species remains constant, a mechanism must be 

 introduced at some stage in the life-cycle by means of which a reduction 

 or halving of the number of the chromosomes takes place. In animals this 

 generally occurs at the formation of the gametes, but in most plants it takes 

 place in the production of non-sexual spores. The significance of the position 

 of this halving of the chromosome number in relation to the life-cycle will 

 be considered under Alternation of Generations in Volume III. 



The mechanism whereby this halving is effected is termed meiosis, 

 from the Greek word meioo = to lessen. In every diploid nucleus half the 

 chromosomes have been derived from one parent and half from the other 

 parent, and hence we can refer to a maternal and paternal set. It has already 

 been pointed out that in all nuclei the size and shape of the chromosomes 



Fig. \^.—Aloe arborescens. Complete diploid set of chromosomes 

 showing homologous pairs. (After Sharp.) 



differ among themselves, hence in a diploid nucleus the chromosomes all form 

 pairs, and the members of each pair are termed homologous chromosomes. 

 In each such pair one member is maternal and one is paternal (Fig. 14). 



The essential feature of meiosis as opposed to mitosis is that, in the meta- 

 phase of the division, instead of split halves of each chromosome passing to 

 the two poles of the spindle, the homologous pairs of chromosomes unite, 

 then come on to the equatorial plane and separate, so that one whole chromo- 

 some of each pair goes to one pole and one to the other. Since it is a matter 

 of pure chance how the chromosomes are arranged at the metaphase, it does 

 not necessarily follow that all the maternal chromosomes pass to the same 

 pole. Usually there is mixing, so that the resulting nuclei contain some 

 paternal and some maternal chromosomes. In no normal case, however, 

 do two homologous chromosomes go to the same daughter nucleus. 



Meiosis used to be regarded as involving two separate nuclear divisions 

 which closely followed one another. Since, however, the first is not normally 

 completed before the inception of the second, it is now considered preferable 

 to regard the whole process as continuous but passed through in two stages, 

 resulting in the production of four monoploid daughter nuclei (Fig. 15). 



