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 Introduction

 In the majority of plants, we can distinguish the succession of two generations; the first generator of gametes and a second which begins with egg formation. These two generations are separated by two important events:

- Meiosis, where the number of chromosomes is reduced.

- Fertilization is where two male and female haploid gametes fuse together to give a diploid zygote which can be at the origin of a new diploid individual.

The succession of generations is called the development cycle, and the importance of each event varies according to the major plant groups.

The life cycle of flowering plants

Angiosperm development cycle

The chromosomal cycle of an angiosperm:
- In the pollen sacs, mother cells undergo meiosis and
produce haploid microspores; and parallel to the level of the ovaries;
mother cells also undergo meiosis and give rise to macrospores
haploid. The leafy plant of the angiosperms, therefore, produces spores; it is a sporophyte (diploid phase).
- The growth and multiplication of spores give rise to gametes. A
microspore generates the pollen grain which is a male gametophyte. A macrospore gives rise to an embryo sac which is a female gametophyte.

- In angiosperms, the haploid phase is reduced to male gametophytes and females which develop on the sporophyte. The chromosome cycle is therefore a diplophasic cycle.

The chromosomal cycle of an angiosperm
image 1: The chromosomal cycle of an angiosperm

The development cycle of gymnosperms

The chromosomal cycle of a gymnosperm:

At the level of the pollen sacs (microsporangia) of the male cones, cells mothers undergo meiosis and produce haploid microspores; and parallel to macrosporangia of female cones; cells mothers also undergo meiosis and produce haploid megastores.

- The leafy plant of the gymnosperms, therefore, produces spores; it's a sporophyte (diploid phase).

- The growth and multiplication of microspores result in pollen grains, the latter give rise to male gametes, so the pollen grain is a male gametophyte.

- The development of the megaspore generates the endosperm which individualizes two or three archegonia. These archegonia give an oosphere. The endosperm is therefore a female gametophyte.

The chromosomal cycle of a gymnosperm
image 2: The chromosomal cycle of a gymnosperm

- The cycle of gymnosperms is characterized by the alternation of one generation sporophytic with a gametophytic generation. The gametophyte develops on the sporophyte. It is a diplophasic cycle.

The development cycle of plants without flowers

The development cycle of algae, the spirogyra 

The chromosomal cycle of the spirogyra:
The spirogyra is in the form of a multicellular filament. The filaments of the opposite sex can mate and form fertilization bridges. The contents cytoplasm of the male cell passes into the cytoplasm of the female cell and merges with its content. The two nuclei of the two cells unite and give rise to a diploid zygote, which enters into slowed life. When the living conditions become favorable, the egg activates and undergoes meiosis to give four haploid nuclei, three of which degenerate. The remaining cell is a haploid zygospore that germinates into a new filament.

The chromosomal cycle of the spirogyra
image 3:The chromosomal cycle of the spirogyra
- The spirogyra cycle is reduced to a single gametophytic phase, meiosis immediately following fertilization. It is a haplophasic cycle. 

Fern development cycle

Le cycle chromosomique de la fougère:
La fougère vit sous deux formes:
- La forme feuillée qui à maturité produit des sporanges où des cellules mères subissent la méiose et donnent des spores haploïdes (n); cette forme est un sporophyte diploïde (2n).
- Le prothalle qui provient de la germination d’une spore, et qui porte des anthéridies, lieu de formation des anthérozoïdes, et des archégones contenant chacune une oosphère. Le prothalle est donc un gamétophyte haploïde (n).

Le cycle chromosomique de la fougère
image 4: Le cycle chromosomique de la fougère

In ferns, the development cycle takes place by alternating two phases of unequal importance: the diploid sporophytic phase (more important), and the haploid gametophytic phase. It is a Diplo-haplophasic cycle.

Moss development cycle

The chromosomal cycle of mosses:
The polycentric lives in humid environments. It has tufts of male feet and of female feet.
The male feet (n) produce antherozoids in antheridia and the female feet (n) produce oospheres in the archegonia, these are gametophytes. Fertilization results in a zygote that develops on the
female gametophyte and gives a diploid sporophyte (2n) at the top of which a sporangium releases haploid spores (n). The germination of the spores gives a tuft of haploid moss.

The chromosomal cycle of mosses
image 5: The chromosomal cycle of mosses

In mosses, the development cycle takes place by alternating two
phases of unequal importance: the haploid gametophytic phase (more important), and the diploid sporophytic phase. It's a cycle
haplodiplophasic.


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