9i 



BOTANY 



PART I 



A 



sperm nucleus (.s7i:) fuse and form the nucleus of the fertilised egg- 

 cell. The cytoplasm and chromatophores of the embryo are derived 

 from the egg-cell alone in the more highly organised plants ; they are 

 not introduced by the male sexual cell. When the spermatozoid, as 

 in animals and in Thallophytes, is provided with a centrosome like 

 body this does not fuse with the centrosome-like body belonging to the 

 ovum. The centrosome-like body of the fertilised egg-cell appears to 

 be derived from that of the spermatozoid only. 



The ultimate limitation of the male sexual product, as introduced 

 into the ovum in Angiosperms, to a nucleus (sperm-nucleus) affords 

 evidence that the nuclei of an organism are the bearers of its hereditary 

 characters. This assumption is further supported by the complicated 

 process by which the process of nuclear division proceeds in the more 

 highly organised plants and animals ; this process ensures at each 



division equivalent halves and therefore the 

 equivalence of the successive generations of 

 nuclei derived in this way. 



The number of chromosomes remains un- 

 changed in the propagation of asexual organ- 

 isms. Sexual reproduction results in a doubling 

 of the chromosome-number. In lower organ- 

 isms the double number of chromosomes thus 

 brought abo^^t is forthwith, at the commence- 

 ment of germination, reduced to the single 

 number by a reduction division (p. 84), and the 

 single number is maintained in the divisions 



in the polar view. The that follow. In the COUrse of phylogcnctic de- 

 chromosomes are grouped in velopment a new generation, the nuclei of 

 pairs, (x IbOO.) 1 • 1 1 1 1 111 



which had the double number of chromosomes, 

 arose, the reduction division being deferred. The nuclei with the single 

 number of chromosomes are termed haploid, in contrast to those with 

 the double number, which are termed diploid. In each diploid nucleus 

 one half of the chromosomes have been derived from the sjoerm-nucleus 

 the other half from the nucleus of the egg ; otherwise expressed half 

 the chromosomes are paternal the other half maternal. In the diploid 

 cells of plants it can not infrequently be shown that the chromosomes 

 constitute pairs {'*), and we may assume that the two chromosomes 

 of each pair correspond to one another. The grouping in pairs is most 

 obvious in those nuclei in which the chromosomes show difierences in 

 size. Two chromosomes of the same size then form each pair, as is 

 seen in the case repi'esented in Fig. 101. In such cases also it is 

 chromosomes of corresponding size that imite to form the gemini of 

 the nuclear plate in the reduction division. These phenomena indicate 

 that the chromosomes of a nucleus are not identical. The union of 

 equivalent chromosomes in the gemini ensures the provision of the 

 daughter nuclei with a chromosome of each kind, when the separa- 



Fi<i. 101. — Young cells from a 

 transverse section of the 

 root-apex of Galtonia candi- 

 cans, showing a nuclear plate 



