ALBERT

Notes: Résumé On discute l'influence d'un champ magnétique circulaire sur la stabilité d'un fluide visqueux et conducteur de l'électricité en mouvement entre deux cylindres circulaires coaxiaux. Supposant que l'espacement entre les cylindres est petit par rapport à leurs rayons, on donne des résultats sur deux sortes de mouvement du fluide basique, produites par la rotation des cylindres et par un gradient de pression autour des cylindres. On démontre que le champ magnétique peut avancer ou retarder l'accrochage de l'instabilité, suivant les circonstances.

Notes: Summary 1. In young endosperms of Trillium and Paris, the mitoses were synchronised. A comparison of the number of mitoses, determined from the number of nuclei, which had taken place in the first 5 weeks after controlled pollination within and between 10 species of Trillium and 2 of Paris, revealed that the rate of endosperm development varied according to the relationship of the plants used. 2. (i) Trillium species were self-sterile, although, in a few cases, slowly developing endosperms formed and later degenerated. (ii) Within species cross-pollinations of Trillium plants gave rise to rapidly growing regular endosperms in which at least 8 synchronised mitoses had occurred in 5 weeks. (iii) After inter-specific cross-pollination, endosperms developed more slowly and frequently showed nuclear abnormalities. These were particularly frequent in endosperms formed by crossing the Japanese T. kamtschaticum with the American T. grandiflorum. No viable hybrid seed was formed. 3. The results of reciprocal crosses between the more distantly related Trillium species differed, but in most cases the endosperm was more retarded than after crossing closely related species. 4. Mitotic abnormalities, which were not restricted to the most slowly developing hybrid Trillium endosperms, included bridges, acentric fragments, lagging chromosomes, “stickiness”, irregular disjunction, micronuclei, giant polymorphic nuclei, and aneuploidy. 5. After crosses within and between Trillium species, 5 wholly or partially aneuploid endosperms were observed. In each case, one maternal chromosome, either B, C, D, or E but not the large A chromosome, was missing or extra. 6. Paris quadrifolia was self- and cross-fertile. When hybridised with Trillium grandiflorum or T. cernuum, it produced no viable seed but when used as the female parent it formed endosperm which, in spite of nuclear abnormalities, developed fairly rapidly for a few weeks. The reciprocal crosses resulted in very little endosperm production. 7. Differences in the rate of development between reciprocal crosses of distantly related species indicate a strong chromosomal control. Failure to form normal endosperms is due to the interacting influence of the paternal genome introduced by the foreign pollen. The degree of interaction depends on how dissimilar the two genomes are and how strongly the male parent is represented in the hybrid nuclei. When the paternal genome is greatly outnumbered by the maternal ones, as in Paris quadrifolia × Trillium (PPPPT) endosperms, development is more rapid than when both are equally represented, as in Trillium x Paris quadrifolia (TTPP) endosperms.

Notes: Summary 1. Low temperature treatment reveals allocyclic segments in Fritillaria recurva (n=12), Tulbaghia pulchella (n=6), and in six species of Cestrum (n=8). These segments, which are underspiralised at mitotic metaphase, correspond to the interphase chromocentres and to the precociously condensed segments at pachytene. 2. The chromosomes of F. recurva contain H-segments in 11 of the 12 pairs. The patterns reveal heterogeneity in 10 of these in the 3 individuals considered, which were heterozygous for 8, 7 and 6 chromosome pairs. The close relationship of F. recurva, F. lanceolata and F. falcata is reflected in the parallel polymorphism of the heterochromatin patterns. 3. The plant of T. pulchella was heterozygous for the H-pattern in 5 pairs of chromosomes. The sixth, the nucleolar chromosome, contained no heterochromatin. The H-pattern was the same in the tetraploid cells which formed up to 80% of all divisions in lateral roots. 4. H-segments were present in 7 pairs in Cestrum elegans, and all 8 pairs in C. parqui and an unidentified Cestrum species. These plants were heterozygous for 4, 1 and 3 pairs respectively. No clear parallel polymorphism was detected in the H-patterns, but single chromosomes showed similarities. An attempt to induce differential contraction by means of chloramphenicol was unsuccessful, the effect being to produce colchicine-type metaphases. 5. Tulbaghia alliacea (n=6) and Hyacinthus litwinowii (n=9), newly described, are both heterozygous for a single segment with a previously unknown type of allocycly. This segment is normally uncontracted throughout the mitotic cycle and becomes fully spiralised only during division after the inhibition of anaphase by low temperature or chemical treatment. This segment is attached terminally to the secondary constriction of only one of the nucleolar chromosomes in each plant. 6. In Fritillaria the segments are mostly proximal and the chromocentres fuse to a considerable extent. In Tulbaghia the segments are terminal and there is little chromocentre fusion. Cestrum elegans, with segments dispersed along the chromosomes, is intermediate in this respect. Fusion appears to be controlled by the proximity in the telophase nucleus, in turn influenced by the position of the segments in the chromosomes. 7. In Fritillaria, Cestrum and Tulbaghia all or most chiasmata are formed near to the centromere. In Fritillaria and Cestrum, where this distribution overlaps that of the heterochromatin, the H-segments are interrupted by small regions of euchromatin. In Tulbaghia, where heterochromatin and chiasmata are localised at opposite ends of the chromosome arms, the segments are entire. It is suggested that crossingover in adjacent euchromatin causes small rearrangements of H-segments. 8. Chromosome maps showing the distribution of H-segments reveal structural heterogeneity within the species and corresponding heterozygosity within the individuals. Heterogeneity of H-patterns involves variation in the size and position as well as the number of H-segments. The frequency of heterozygosity in an individual does not necessarily increase with the number of segments.

Notes: Summary A proof is given of theBott periodicity theorems using only well known techniques of algebrdbc topology.