96 BOTANY part i 



spores by a process of multicellular formation (p. 90). Fig. 102 will 

 serve to make this process clearer. The four diagrams, A «-(/, give 

 the scheme of the process as it happens in the pollen mother cell of 

 the Lily ; the natural appearance of this was shown in Fig. 88. In 

 A a the gemini are on the nuclear spindle and a longitudinal split is 

 seen in each component chromosome. In A b the chromosomes are 

 separating, the V-shape being given by the separation of their longi- 

 tudinal halves. In A c, the mother cell has divided and on the two 

 nuclear spindles these longitudinal halves of the chromosomes are seen 

 associated together. The separation of these longitudinal halves in A d 

 shows that the homotypic division is one of the nucleus into equivalent 

 nuclei. The division of the two nuclei here, as in Fig 88, is shov/n 

 taking place in the same plane. The figures B a-d are only slightly 

 diagrammatic. In a four grand-daughter nuclei are seen in the same 

 plane with the septa forming between them ; this leads to the arrange- 

 ments of the spores represented in B b. The grand-daughter nuclei 

 in B c are tetrahedrally arranged, so that only three of them are visible 

 at the one time. The partition walls have formed between them. 

 This leads to the tetrahedral arrangement of spores shown in B d. 

 The figures B b and B d further show how the protoplast of each spore 

 surrounds itself with a special Avail, differing in structure and chemical 

 nature from the previous wall. The special membrane remains en- 

 closing each spore while the old wall disappears. 



Although it is by fertilisation that the further development of the 

 sexual products is made possible, there are exceptional cases in Avhich 

 a gamete or an egg forms an embrj^o without being fertilised. If 

 development follows in this way from a gamete or egg with the 

 reduced number of chromosomes, the phenonemon is called true 

 PARTHENOGENESIS. This is as yet only known in the lowest groups 

 of plants, especially among the Algae, e.g. in Chara crinita belonging 

 to the Characeae. On the other hand the reduction of the number of 

 chromosomes may be omitted in the life-history of the plant ; in place 

 of a haploid generation with haploid nuclei a generation otherwise 

 similar but with diploid nuclei and forming eggs with diploid nuclei 

 develops. When such an egg, which already possesses the double 

 number of chromosomes, usually 1)rought about by fertilisation, proceeds 

 to form an embryo without being fertilised, we have to do not with 

 true parthenogenesis but with a development process due to loss of 

 sexuality or apogamy. The cases recorded for numerous families of 

 the more highly organised plants (Coinpositae (^-), Rosiflorae ('''^) 

 Ranunculaceae (■*^) Tbymelaceae (*''), Urticaceae ("") and JNIarsiliaceae 

 (^') ), are of this nature. 



In certain cases fertilisation is replaced by a fusion of nuclei of 

 adjoining tissue cells of the haploid generation and the product 

 of fusion gives the starting point for the development of a diploid 

 generation. This hajjpens in certain varieties of Ferns {e.g. of 



