84 BOTANY part i 



pole. In ail cases the separation pi-oceeds from the point of attachment of the 

 traction-fibres. AVhen a paired chromosome is attached to the spindle near one of 

 its ends, the separation of the daughter chromosomes naturally commences near 

 this end ; when the attachment is by the middle of the chromosome the daughter 

 chromosomes remain longer attached by their ends. In Fig. 86 the behaviour of 

 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 PKOPHASES 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 metaphase. The separation of the 

 daughter chromosomes is accomplished in the anaphase, 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 sjjlitting (Fig. . 86, 5, 6, 7 ; 

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

 repetition of the prophases. The reversal of the stages in the process of nuclear 

 division commences with the separation of the daughter chromosomes. The stage 

 of the nuclear plate at which the progressive is replaced by the regressive move- 

 ment tends to last a considerable time. 



The number of chromosomes occurring in any nucleus is a definite one, and when 

 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 chromosomes of a nucleus may be 

 of ditferent sizes ; when such differences in size exist they persist in successive 

 divisions. The smallest number of chromosomes 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 Avith 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 marked contraction of the nuclear contents, Avhich is char- 

 acteristic 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 j'et 

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

 corresponds to one paired chromosome. The luimber of these segments 

 which are termed GEMINI is half as great as the number of chromo- 

 somes 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 peripheiy of the 

 nucleus ; this is the condition that has been termed diakinesis (8). 

 At this stage kinoplasmic filaments are becoming ap})lied to the 

 nuclear membrane (8) ; the latter disappears and the nuclear spindle, 

 which is at first multipolar (9), but ultimately becomes bipolar (10), 



