CHROMOSOME NUMBERS AND MEIOSIS IN CERTAIN SALICES

constriction (subterminal centromere).Most diploid species have 4 L', 4 T and 2 S, the idiogram being 4 L' +28 M +4 T+2 S.Thanks to the activity of British cytologists the chromosome numbers of all native species (20) in the new Flora of the British Isles are known.MARKLUND has determined the chromosome numbers of the majority of Scandinavian species, his numbers being published in the Scandinavian Flora of HOLMBERG and in chromosome lists of LOVE and LOVE.Our knowledge of Central European species is fragmentary where species lacking a wide distribution are concerned.Japanese investigators have recorded a number of Salices from Eastern Asia.The numbers of many Salix species are unknown: most of them are in subtropical regions or other regions where cytological investigations do not seem to occur.The highest degree of polyploidy that was found is octoploid (S. glauca, S. borealis) and decaploid (S. myrsinites).Intraspecific polyploidy is known in Salix.The first case reported was S. aurita, which is diploid and tetraploid.Later diploid and tetraploid cytotypes have been found in S. lapponum and in S. caprea, and diploid and decaploid cytotypes in S. myrsinites.In S. caprea MARKLUND found a triploid; a similar shrub was found by WILKINSON in the usually diploid S. daphnoides.WILKINSON stresses that there are no taxonomical differences between the different cytotypes (see also Flora of the British Isles).Surely in Salix we have true intraspecific polyploidy!My own contributions to Salix cytology are based on the experimental material of Professor HERIBERT NILSSON (HAKANSSON, 1929, 1933, 1938, 1944).The good chromosome pairing at first metaphase and the regular first anaphase of simple and complex hybrids with an even number of genomes were a striking fact.This general regularity of meiosis did not, however, exclude the formation of diploid or polyploid pollen grainsin S. viminalisXS.caprea in a frequency of about 1 % of functioning d'-gametes, to judge by the number of triploids in the progeny.In the crosses shrubs with unexpected and striking exterior, which NILSSON called »extravagant forms», had occasionally appeared.They were interpreted by him in a purely Mendelian way as extreme recombinations.However, several extravagant forms were polyploid and thus genome mutations rather than recombinations; instances are the triploid gigantea and [alciiolia types, the tetraploid neocinerea, and the pentaploid laurina, all of them having appeared in the F. generation of S. viminalisXS.caprea.Another experimental polyploid was the extravagant form amerinoides from the cross S. (viminalisXS.repens) X S. repens; it was tetraploid.

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