As with the majority of plants, Atropa belladonna exhibits an alternation of generations. This basically means that the plant alternates between a multicellular haploid (cells have one set of chromosomes) stage and a multicellular diploid (cells have two sets of chromosomes) stage. Basically, the "adult" plant produces spores via meiosis, then those spores develop into a multicellular haploid plant called a gametophyte. Then, the gametophyte undergoes mitosis, forming haploid gametes, which grow into the "adult" plant, or sporophyte. Then the process repeats, and so on and so forth.

In greater detail, the life cycle of Atropa belladonna is as follows:

In the male reproductive structures (anther), haploid microspores are formed when microsporocytes undergo meisos. Simultaneously, in the female reproductive structure (ovule), four haploid megaspores are formed by one megasporocyte undergoing meiosis. Only one survives.

Next, each microspore undergoes mitosis to form a pollen grain, which is the male gametophyte.

The megaspore also undergoes mitosis to form seven haploid cells: one contains two nuclei (polar nuclei), and one of the cells becomes the egg. The entire structure is the female gametophyte.

Next, via either the wind or insects, a pollen grain reaches another flower, and makes a pollen tube to the ovary, where the female gametophyte is waiting.

 Then, two sperm from the microspore travel down the pollen tube to the female gametophyte. One sperm fertilizes the egg cell, which forms the diploid zygote. The other sperm fertilizes the two polar nuclei,  which forms a triploid cell. This process of two sperm separately fertilizing the egg cell and the polar nuclei is called double fertilization, and is unique to Angiosperms. The zygote becomes the embryo, and the triploid cell becomes the endosperm, which serves as nutrition for the developing embryo inside the seed.

Then, the seed is dispersed somehow, grows into a sporophyte, and the cycle begins again.

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