The chromosome constitution of the pomoideæ

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Abstract
The following numbers were found: (1) Diploids: n = 17 in Pyracantha coc-cinea lalandei, Cydonia (Chaenomeles) cathayensis, C. japonica, C. maulei, Pyrus communis vars. "Williams'' Bon Chretien" and "Beurre Giffard," P. malus vars. "Rev. W. Wilks" and "Baumann''s Reinette," P. tormi-nalis, P. sorbus, P. aria, P. hybrida, Rhapiolepis de-lacourii, R. indica, Stranvaesia davidiana undulata, Mes-pilus germanica, Crataegus oxyacantha, C. chlorosarca, and C. monogyna; 2n=34 in Pyrus communis var. "Fertility." (2) Triploids: 2n=51-52 in Cotoneaster congesta, Pyrus malus var. Washington, P. minima, and Crataegus monogyna var. cabulica. (3) Tetraploids: n = 34 in Cotoneaster vulgaris, C. bullata, Pyrus arbuti-folia, Photinia villosa, Amelanchier canadensis, A. laevis, Crataegus cognata, C. pruinosa, C. apposita, and C. pedicillata. The basic number in the Pomoideae is 17. Secondary pairing occurs amongst the chromosomes of all the "diploid" types, forming sexivalent and quadrivalent groups. In extreme cases 7 groups are formed,[long dash]3 sexivalents and 4 quadrivalents. Multivalent associa-tions occur less frequently. Two types of triploids occur: (1) auto-triploids. These usually form trivalents, but higher associations have been observed, showing that auto-syndesis takes place within each of the supposed haploid complements; divisions are slightly irregular. (2) Allo-triploids, formed from a cross between a diploid and a tetraploid, and having very irregular divisions with numerous univalents. Tetraploid forms show secondary pairing, groups of 6 and 8 chromosomes being most frequently found. Seedlings of triploid X diploid apples most frequently had 2n+7 chromosomes instead of showing a binomial frequency or elimination of intermediate numbers. In triploids intercrossed or selfed the seedlings showed a higher chromosome number than expected. Chromosome pairing, chromosome morphology and breeding results indicate that, as in Pyrus, the 34 chromosomes of the diploid Pomoideae are of 7 types, of which 4 are represented 4 times and 3 are represented 6 times. The number 17 is therefore a secondary basic number (unbalanced relative to the primary basic number) and the derived series of polyploids (2n = 34, 51, 68) are secondary polyploids. On analogy with experimental observations, the establishment of a secondary basic number must mean a definite evolutionary step. It is plausible, therefore, that the Pomoideae owe their special morphological characters to this state of secondary balance, indicating that change of balance is a factor in evolutionary change.

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