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Biology/Alternating generations

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Question
Hello,

Please can you help me, I am trying to figure out plant reproduction and think Iíve got the basics but Iím confused about the idea of Alternating Generations.

What I understand so far is that first a cell within the plantís sexual organs divides by meiosis to become two haploid cells, the gametes, either male or female. The male gamete leaves the plant via pollen, finds a female flower/female part of flower and fuses with a female gamete to create a diploid zygote, which then gets bigger, via mitosis to form an embryo contained within a seed, which then leaves the parent plant and grows into a new plant. So far so good, I hope.

What I don't understand is that this process seems very similar to animal reproduction Ė haploid sperm/pollen cell leaves the male, fuses with haploid egg to form a diploid zygote, but in plants this is described as involving an Alternation of Generations, one generation haploid, the other diploid, but in animals it is not.

I can see that some plants have clear Alternate Generations (eg moss) because two completely different plants grow, the sporophyte and the gametophyte, but for most plants this is not the case. So if in plants the two generations are simply: haploid gamete followed by diploid zygote, why does the description Alternating Generations not also apply to animals that also have a haploid gamete state and a diploid zygote stage?

The only possible difference I've been able to figure out is that in animals the haploid stage is only ever unicellular, where as maybe in plants it is multicellular, is that it?

If you can explain that would be fantastic, because I just canít figure it, thank you.

Answer
Hi Petra
Your conclusions are actually correct. An  adult animal or a tree represent the Diploid or sporophyte Generation. During meiosis the gametes are produced and these are the gametephyte generation. In higher plants and animals only the gametes make up the gametophyte generation. Once fertilization occurs the zygote is diploid and a sporophyte. In animals the term adult is used instead of a sporophyte. In bryophytes and Fungi there is actually a multicellular plant the produces gametes.  

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Walter Hintz

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Science teacher for over 50 years. MSc. in biology. I can answer questions in general biology, zoology, botany, anatomy and physiology and biochemistry.

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I have a MSc in biology and have been a science teacher for over 50 years. At present I am a faculty member at a college and a science consultant at seven catholic schools.

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The Ohio journal of Science
Momentum-The Journal of the Catholic Education Association

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