Section 6 — Cytology 



contracted metaphasic plates before the separa- 

 tion during second division. 



To prove the validity of the described type of 

 meiosis in a material with a heteromorphic 

 bivalent, the meiosis of plants with one broken 

 chromosome have been studied. One hetero- 

 morphic association is then formed in each 

 meiotic cell, originating from the pairing between 

 the two fragments and their homologous un- 

 broken partner. At first metaphase this asso- 

 ciation is always an open chain with the large 

 chromosome in the middle of the association 

 and the two small ones on each side. This 

 association separates equationally during first 

 anaphase giving one long and two short chro- 

 matids on each side of the equator. At inter- 

 kinesis the two fragments pair again with their 

 long partner, the result of which can be studied 

 at the end of the separation during second 

 telophase. In the four cells of the tetrad the two 

 small chromosomes regularly substitute the origi- 

 nally broken chromosome, giving rise to four 

 cells, two with three large chromosomes and two 

 with two large and two small ones. 



facilitated by the knowledge that the organisms 

 are diploid as shown by the cytological evidence, 

 and not haploid as previously assumed. 



6.34. Meiosis in the Sex Organs of the Biflagellatae. 



E. Sansome (Zaria, North Nigeria). 



Critical cytological evidence showing that 

 meiosis occurs in the oogonia and antheridia is 

 given for Pythium debaryanum, Phytophthora 

 cactorum and an Achlya sp. In the case of 

 Pythium debaryanum (n = 18 ca.) an association 

 of four chromosomes was observed in metaphases 

 in the sex organs. In the case of Phytophthora 

 cactorum (« = 9 ca.) and Achlya (n = 8 ca.) 

 multivalents were observed in polyploid nuclei 

 in the oogonia and antheridia after treatment 

 with natural camphor. Additional evidence in the 

 case of Phytophthora cactorum was the observan- 

 ce of a bridge and fragment at anaphase, 

 evidently resulting from crossing over in an 

 inversion. 



The fact that two successive divisions occur 

 and that the size of the nuclei at the end of the 

 two divisions is approximately half that of the 

 vegetative nuclei is in full accordance with the 

 other cytological evidence in indicating that 

 these two divisions constitute meiosis. 



The finding that meiosis occurs immediately 

 before and not after fertilization has important 

 genetical implications. A survey of the scanty 

 genetical data in this group shows that they can 

 be more readily explained on the basis of the 

 organisms being diploid rather than haploid. 

 Further genetical investigations should be 



See Sansome, E., Nature, 191, 827 (1961); 

 Sansome, E., and Harris, B. J., Nature, 196, 

 29-292 (1962); Sansome, E., Trans. Brit. Myc. 

 Soc. in the press. 



6.35. Which are the Specific Biocatalizers for the 

 Multiplication of Chromonemata and, conse- 

 quently, of the Genes or of the DNA Molecules 

 in Higher Plants? F. Resende (Lisbon, Por- 

 tugal). 



Growth in a pericellular plant is the result 

 of the growth of its constituent cells. This 

 growth is till now considered as of 2 types: 

 (a) cell multiplication and (b) membrane disten- 

 sion. 



In some species of the succulent genera 

 Bryophyllum and Kalanchoe, growth is extremely 

 sensitive to photoperiodism, being much greater 

 under LD (long day) conditions than in SD 

 (short day). This LD effect results from a much 

 enhanced growth of the stem internodes, of 

 petiole length and of leaf area. The amount of 

 growth biocatalizers is consequently greater in 

 LD than in SD, IAA (3-indolyl-acetic-acid) 

 concentration, for instance, may be 20 times 

 greater (Linskens and Resende). 



In plants with arrested growth (i.e. in rosette) 

 through SD action, nuclei are, however, larger 

 than in plants under LD and showendopolyploidy, 

 which may reach 32 x, whereas under LD 

 nuclei never show more than 8 x (Von Witsch 

 and Fliigel). 



It all seems to function as if biocatalysis govern- 

 ing cell multiplication and cell membrane 

 distension does not affect the growth and the 

 multiplication of the chromonemata and, hence, 

 the synthesis of genematerial, DNA, and protein. 

 This synthesis in LD is at most equal (proba- 

 bly even inferior) to that in SD, and therefore 

 not related to the general plant growth bio- 

 catalizers known up to the present. Hence, it is 

 logical to postulate the existence of specific 

 biocatalizers for the growth and multiplication of 

 the chromonemata, and which are independent 

 from those governing multiplication of the nucleus 

 and cell and the distension of its membrane. This 

 hypothesis further explains the cases of ectomi- 

 tosis (Resende), i.e. nucleus multiplications 

 without multiplication of chromonemata. 



112 



