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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    evolution of the life histories of ])lants. 'I'he nucleus of every living cell contains a definite number of chro- mosomes, bodies that are supposed to represent the physical basis of heredity, in the same sense that pro- to})lasm rej)resents the “physical basis of life.” In other words, they are thought to be connected in some way with the reactions that transmit the characters of a parent to its jirogeny, d'he number of chromosomes is definite for each kind of plant and animal, and the characteristic number appears in ever>' cell of the body. In different organisms the number ranges from two to more than a hundred. It is obvious that when two gametes fuse, the zygote contains the chromosomes of both; that is, the double number, as it is called. If the characteristic gamete number is 12, the zygote contains 24. Whenever the sex act (fertilization) oc- curs, therefore, the number of chromosomes is doubled. When the zygote produces a new individual, each cell of that individual contains the double number. Unless something should prevent, the chromosomes would continue to double with each succeeding generation, ever>’ time fertilization occurred, soon resulting in an impossible number. Sexual reproduction, therefore, necessarily involving a doubling of the chromosomes, must also involve a provision for reducing the number again. The life history of every sexual plant and ani- mal, therefore, includes two principal events, namely, fertilization, which doubles the chromosomes, and some other process which reduces the number again. The place of reduction in the life historj' varies, but in general among plants it occurs in connection with the formation of spores, which thus contain the half-