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In my texbook it is written that for non- vascular seedless plants the dominant generation is the gametophyte as the gametophyte generation is larger and longer lived.
Is the criteria for dominant generation?-( Which generation the plant spends most of its life in?)

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2 Answers 2

The wikipedia page Alternation of Generations actually has a pretty sophisticated discussion of this issue, and discusses the fact that plants can and do fall at a lot of different places on a gradient of one generation being dominant over another.

I think the root of this complexity is that this is an issue of terminology, and is connected to your concept of "what is an organism", which is a complex issue in and of itself (see this answer).

Despite this, I think that you can still usefully discuss use a number of criteria which argue for a generation being more or less dominant, including generation time, but also:

1) size (as you say in your question)

2) morphological complexity

3) ability to independently undergo "life" processes like photosynthesis and mitosis

4) conspicuousness/salience to humans (as we're the ones making up these categories)

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I should also link to @Remi.b's answer to OPs other question, which is very similar: biology.stackexchange.com/questions/14186/… –  Oreotrephes Jan 4 at 21:55
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According to "Wile, Dr. Jay L. Exploring Creation With Biology. Apologia Educational Ministries, Inc. 1998" and reported here,

In alternation of generations, the dominant generation is the generation that occupies the largest portion of the life cycle

I guess that it has to be understood in terms of "time". I am not quite sure though if it has to be understood in terms of generation time of life span. Intuitively, I would say that generation time would make much more sense. Therefore, I'd say that, the dominant generation is the generation which have the longest mean generation time (=mean time from the birth to reproduction).

As @Shigeta said in the comments: For example, in Homo sapiens, the dominant generation is the diploid phase (sporophyte, you and me) and the non-dominant generation is the haploid phase (gametophyte, sperm/ovule).

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an example is the human animal, which is dominant over the gamete (sperm/egg) generation. –  shigeta Dec 31 '13 at 15:52
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