26 



A TEXTBOOK OF THEORETICAL BOTANY 



any nuclear division and may be given a name or number. Its history can 

 thus be traced through the development of the individual and from generation 

 to generation. There is no escape from the conclusion that the chromosomes 

 are persistent entities, despite the fact that they are not usually recognizable 

 as individuals in the metabolic reticulum, though their persistence in that 

 state may be inferred from the number and distribution of the chromocentres. 

 It is important to realize that in every diploid nucleus there are two sets 

 of chromosomes, one derived from the male parent and the other from the 

 female. It follows that, in all normal cases, the diploid number of chromo- 

 somes must be even. There is thus a pair of chromosomes of each 

 type, called homologous chromosomes. Unpaired chromosomes are of 

 exceptional occurrence. In hybrids, in cases where the chromosome com- 

 plements of the two parents are not identical, some chromosomes may be 

 unpaired, and if there are many of these the cell cannot survive, that is to 

 say, the union of the parents is infertile. In some species there are also 

 unpaired sex chromosomes, concerned with the determination of sex. 

 As the single set of chromosomes, derived from one parent, contains the 



complete set of that parent's genes it 

 is called the genome. 



The time relationships of the 

 phases in mitosis are fairly constant, 

 though the total time occupied and 

 also the interval between successive 

 mitoses depend not only on the 

 type of tissue but on external factors 

 such as temperature. Mitosis can be 

 watched in living cells in some cases, 

 such as hair cells or the stigma cells 

 in grasses. In the stigmas of Arrhe- 

 natherum at 19° C. the total time for 

 the process was found to vary be- 

 tween 80 and no minutes. The 

 relative duration of the main phases 

 is shown in the diagram (Fig. 11). 

 Those phases which last longest will obviously be those most often seen in 

 stained preparations. 



Fig. II. — Diagram to illustrate the time 

 relationships between the phases in a 

 complete mitotic cycle. 



Cell Division 



During telophase spindle fibres remain visible, bridging the space between 

 the daughter nuclei and forming a somewhat barrel-shaped group. Whether 

 they are in fact the same fibres as those attached to the chromosomes, or a 

 newly organized set, is not certain, but the latter is more probable. 



Across the equator of this set of fibres a transverse cleavage develops, 

 dividing the cytoplasm and cutting across the fibres. This fissure widens 

 and appears to contain fluid, which soon solidifies as the result of the 



