Evolution of sex in plants / by John Merle Coulter.

  • Coulter John Merle, 1851-1928.
Date:
[1914]
Catalogue details

Licence: In copyright

Credit: Evolution of sex in plants / by John Merle Coulter. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    plants, but it does not take into account the underlying causes of the alternation, d'here are cytological features in this alternation that are more fumlamental than the appearance and behavior of the two kinds of individuals. As stated in a preceding chapter, the number of chromosomes in the nucleus is a constant feature of each kind of plant and animal. When two nuclei fuse in the act of ferti- lization, the nucleus of the zygote contains the double number of chromosomes, and this double number is passed on by successive cell divisions until it is reduced again, which in ])lants usually occurs in connection with spore-formation. This means that in the life history of a plant in which sexual reproduction takes place, the chromosomes occur in two numbers. For convenience, these two numbers are usually indicated as x and 2X. d'racing the chromosome situation through the life history of a plant, the following facts appear, 'I'he zygote or fertilized egg is, of course, a 2X cell. In the alternation of generations the zygote produces the sporophyte, which is therefore a 2X individual. When the sporophyte produces spores the reduction of chromosomes occurs, so that the spores are x cells. In the alternation of generations the spore produces the gametophyte, which is therefore an x individual. For this reason, the sporophyte is usually spoken of as the 2X generation and the gametophyte as the x generation. It follows that 2X cells belong to the sporophyte and X cells to the gametophyte; therefore, the zygote is the first cell of the sjiorophyte, and the spore is the first cell of the gametophyte. If the chromosome test be applied in determining the two generations, it becomes obvious that alternation