206 



BOTANY 



PART I 



shaded and united in three gemini. The two lateral gemini are seen from the 

 side, the middle one from the end. The latter one shows the longitudinal split in 

 the component chromosomes and the orientation of the plane of fission. In B b 

 the chromosomes of each geminus have separated and are moving towards the poles 

 of the spindle to form the two daughter nuclei. The two halves of each chromo- 



B 



FIG. 234. Diagrammatic representation of ordinary nuclear division (A) and of the reduction 

 division (B). (After STRASBURGER.) 



some thus go to the same daughter nucleus. This division results in a reduction 

 of the chromosome number from six to three. In contrast to this reduction 

 division, which, because whole chromosomes separate, results in a definite differ- 

 ence of the products of division, may be placed the somatic nuclear division. 

 This, since the longitudinal halving of the chromosomes gives rise to completely 

 equivalent products of division, may be termed equation division. 



Opinions are divided as to how and when the chromosomes in the reduction 

 division become associated in pairs, or temporarily united in a single structure. 

 It is possible that the scheme of the reduction division is not always the same. 

 Usually the chromosomes appear placed side by side in the pair (parasyndesis), 

 but in some cases they appear to be placed end to end (metasyndesis). 



SECTION IV 



THE THEORY OF DESCENT AND THE ORIGIN OF NEW SPECIES 



A. The Theory of Descent ( 104 ). How the organic forms living 

 on the earth with their morphological peculiarities have arisen is one 

 of the most important theoretical questions in morphology. The 

 assumption once made that the kinds of plants were independently 

 created (theory of special creation) has become gradually abandoned 

 in favour of a theory of evolution, especially owing to the deepen- 

 ing of morphological knowledge and the influence of CHARLES 

 DARWIN. This has already been referred to in the Introduction. 

 The theory of evolution regards the existing organisms as developed 

 from other and frequently more simply-constructed forms which lived 

 in earlier periods of the earth's history (cf. p. 1 ff.). This fundamental 

 biological theory now permeates morphological investigation so 

 completely that it is indispensable for the morphologisfc to be 

 acquainted with the evidence for it. Evidence is afforded by classifica- 



