The life cycle of most plants is referred to as alternation of generations and is characterised by alternating diploid and haploid stages. The diploid generation reproduces through spores (hence the name sporophyte). The spores develop into the haploid generation, which produces gametes (hence the name gametophyte). When two gametes fuse, a zygote is formed, which develops into a new diploid generation, and the life cycle starts again.
The sporophyte generation receives one set of chromosomes from both the male and female gametes, that is, this organism is diploid. The sporophyte organism produces haploid spores through meiosis (or reductive cell division). The gametophyte generation that develops from it is also haploid.
The life cycle of mosses
mature moss plant
young moss plant
male 'reproductive organ' (♂)
female 'reproductive organ' (♀)
sporophyte (diploid) generation
gametophyte (haploid) generation
Mosses are the simplest land plants: they do not have differentiated tissues or real organs. The undifferentiated vegetative tissue of these organisms is called a thallus. The mature moss plant is the gametophyte generation, a haploid organism.
This organism has primitive male and female 'reproductive organs' (which are not real organs either) that produce male and female gametes (sperm and egg cells). The presence of water is necessary for fertilisation to take place: in the water, sperm cells can swim to the female gametes found in the female 'reproductive organ'. This is a type of chemotaxis, as the movement of the sperm cells is a response to a chemical stimulus originating in the direct environment of the egg cells.
The fertilised the egg, that is, the zygote, develops into the diploid sporophyte generation, that is, a seta and a sporangium. When the sporangium matures, it opens, and the spores, which were produced in it through meiosis, spill out onto the soil. They then develop into a new gametophyte generation, that is, a haploidmoss plant.
In the case of mosses, the gametophyte generation is more developed, and therefore more dominant than the sporophyte generation.
The life cycle of ferns
cluster of sporangia (sorus)
female reproductive organ (♀)
male reproductive organ (♂)
developing fern plant
Ferns are vascular plants, that is, they have real tissues and organs. The fern plant is the sporophyte generation. Haploid spores are produced through meiosis in the sporangia, which are organised in clusters (sori) on the back of the leaves. When the sporangia open, the spores spill out onto the soil then develop into a prothallus, which is the gametophyte generation; it has male and female reproductive organs. In the presence of water, sperm cells can swim to the female reproductive organs, where they fertilise the eggs and thus zygotes are formed. Zygotes then develop into diploid fern plants and the prothallus disintegrates.
In the life cycle of ferns, the sporophyte generation is much more developed, and therefore dominant, than the gametophyte generation.
The life cycle of gymnosperms
seed cones (♀)
pollen cone (♂)
pollen grain with air pocket
The life cycle of gymnosperms is similar to that of ferns in that the sporophyte generation is more developed. Water, however, does not play a role in the fertilisation process of gymnosperms, and they are heterosporous, that is, they have two different types of spores, male and female. These are produced on special reproductive structures called carpels, that form the separate male and female cones.
Female spores, called megaspores develop in the ovules found at the base of the carpels of female cones. Four megaspores are produced in each ovule, and an egg cell is produced by one of these megaspores.
The carpels of male cones contain microsporangia. In these, microspores are produced through meiosis. Microspores then develop into pollen grains. Sperm cells develop from one of the cells of the pollen grain. During pollination, pollen grains are transported to the female cones by wind and some of them land near the opening of the ovule. Pollen tubes emerge from these pollen grains and grow through the ovule towards the egg cell. The sperm cells travel through the pollen tube and one of them eventually fertilises the egg. After fertilisation takes place, a winged seed develops from the zygote and may develop for as long as 3 years. When it matures, it falls out of the female cone onto the soil and starts to develop into a new plant.
The flowers of angiosperms provide more protection for gametes developing inside than the cones of gymnosperms. This is because reproductive cells do not develop on the surface of open carpels but inside closed ovaries and anthers.
The ovary, found at the bottom of the pistil (the female carpel), contains an ovule. In the ovule, meiosis takes place and 4 megaspores are produced. Only one of these is functional; its nucleus undergoes mitosis and forms one large cell with 8 nuclei. The nucleus closest to the opening of the ovule becomes the egg. The two polar nuclei found in the centre form the central cell.
In the male carpels, called anthers, male spores, called microspores are produced through meiosis. As the microspores mature, their nuclei undergo mitosis and form a tube nucleus and a generative nucleus. At this point, the microspore is a pollen grain. In a pollen grain, the generative nucleus undergoes meiosis and produces two sperm cells.
When a flower is pollinated, a pollen grain falls onto the surface of the pistil. A pollen tube emerges from the pollen grain and extends towards the ovule. Two sperm cells travel through the pollen tube to the the ovule and one fertilises the egg, while the other fertilises the central cell. This is called double fertilisation.
After fertilisation takes place, a fruit with a seed inside it starts to develop. When the seed germinates, the development of a new angiosperm plant starts.