84 BOTANY PAKT i 



the chromosomes is represented as in the diagrams 3 and 4 (Fig. 87). As a rule it 

 does not appear so clearly, but more or less combined with the other type. 



The changes occurring in a mother nucleus preparatory to division are termed 

 the PROPHASES of the karyokinesis. These changes extend to the formation of the 

 nuclear plate, and include also the process of the longitudinal division of the chromo- 

 somes. The stage of the nuclear plate is the METAPHASK. The separation of the 

 daughter chromosomes is accomplished in the ANAPHA.SE, and the formation of 

 the daughter nuclei in the TELOPHASE of the division. The real purpose of the 

 whole process is attained in the quantitative and qualitative division of the 

 chromosomes, resulting from their longitudinal splitting (Fig. 86, 5, 6, 7 ; 

 Fig. 87). The anaphases and telophases of the karyokinesis are but a reverse 

 repetition of the prophases. 



The number of chromosomes occurring in any nucleus is a definite one, and \vla-n 

 a deviation from the usual number is met with, it is due to some of the chromo- 

 somes having remained united end to end. The smallest number of chromosome 

 which has yet been found in the nuclei of vegetative cells of the more highly 

 organised plants has been eight ; as a rule the number is larger, amounting often 

 to several times this number. 



A special type of nuclear division, to which the name of REDUCTION 

 DIVISION is given,, is met with in those reproductive cells which start a 

 new generation, such as the spore-mother-cells of the higher Cryptogams 

 and Phanerogams. In the prophase of this division the chromosomes 

 become united in pairs (Fig. 88, 1, 2), and there then occurs a mai'ked 

 contraction of the nuclear contents, which is characteristic of this 

 process of division and is called SYNAPSIS (3). After this the double 

 chromosomes become again loosened out as a delicate double thread 

 (4), which soon unites to a correspondingly stout thread, forming a 

 loose skein (5). The doubled nature of this thread soon becomes 

 recognisable again (6). The skein consisting of the as yet unbroken 

 double thread now falls into segments (7), each of which corresponds 

 to one paired chromosome. The number of these segments is half 

 as great as the number of chromosomes in the tissue cells of the 

 same plant, since two chromosomes are represented by each segment. 

 The paired chromosomes become shorter and thicker and are 

 distributed around the periphery of the nucleus ; this is the con- 

 dition that has been termed diakinesis (8). At this stage kinoplasmic 

 filaments are becoming applied to the nuclear membrane (8) ; the 

 latter disappears and the nuclear spindle, which is at first multipolar 

 (9), but ultimately becomes bipolar (10), originates from the kino- 

 plasmic fibres. The paired chromosomes become attached to the 

 fibres of the spindle and arranged in an equatorial nuclear plate (10). 

 Shortly afterwards the separation of the chromosomes, until now 

 united in pairs, takes place (11). In this process, in which the 

 essential of the reduction division is effected, it is not longitudinal 

 halves of chromosomes but entire chromosomes which separate from 

 one another. The result of this is that each daughter nucleus receives 

 only half as many chromosomes as were found in the tissue cells of the 



