Abstract
THE interesting announcement of Dr. J. W. Heslop Harrison (NATURE, January 9, p. 50) that he has found a single pair of sex chromosomes in certain polyploid species of Salix will, if confirmed, add to our knowledge of sex chromosomes in diœcious plants, but does not necessarily alter any of the current views regarding the origin of polyploidy. The fundamental fact that the chromosomes in many plant genera occur in arithmetical series, shows that the process of multiplication in number is one that affects all the chromosomes simultaneously. This is true however the change comes about, whether, for example, through a suspended mitosis or by the union, in hybridisation, of nuclei the chromosomes of which are sufficiently incompatible to be unable to pair in the following meiosis. Multiplication of the whole chromosome series through a condition involving a suspended mitosis is well known to occur naturally in various plant tissues and has been produced experimentally by a variety of methods.
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