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.

    93/160 (page 79)
    In some cases there may be three individuals involved in this alternation; namely, an individual bearing male organs, another bearing female organs, and a third bear- ing sporangia. The male and female individuals together produce the zygote, which produces the spore-bearing individual, whose spores produce the male and female individuals. The variable expression of alternation among the thallophytes, ranging from alternating numbers of chromosomes to distinct alternating individuals, passed into an established and obvious alternation with the appearance of the liverworts, the first land plants. It is usually stated that alternation of generations became established among the liverworts, but it must be under- stood that this refers to obvious alternation, for alternation really became established when sex was estab- lished. It is j)robably true that the land habit, with its much more variable conditions, emphasized alter- nation and made the alternating individuals dilTer. A brief consideration of the situation will e.x]ilain this statement. The favorable condition for the gameto- phyte is moisture, so that fertilization may be accom- plished by swimming sperms; while drier conditions favor the sporophyte, with its aerially disjiersed spores, d'he spores are produced, therefore, in conditions of relative dry ness, but they germinate in moisture, which is the condition for gametophyte development and functioning. .Mternating conditions of dryness and moisture, therefore, would favor these alternating indi- viduals, and make them more and more unlike. An outline of the life history of a moss, which is the same as that of a liverwort, will serve to fix clearly in