Plants are very successful organisms, growing in almost every environment on Earth. Part of their success is due to the fact that they can reproduce both asexually and sexually.
When plants reproduce asexually, they use mitosis to produce offspring that are genetically identical to the parent plant. The advantage of asexual reproduction is that it allows successful organisms to reproduce quickly. The disadvantage is that all the offspring are genetically identical, which decreases the ability of the population to survive changes in the environment.
When plants reproduce sexually, they use meiosis to produce haploid cells that have half the genetic information of the parent (one of every chromosome). Eventually, the haploid cells produce eggs and sperm that combine to create a new, genetically unique diploid organism that has two of every chromosome.
The advantage of sexual reproduction is that it creates diversity in offspring, increasing the chances that the species will survive in the face of environmental change. The disadvantages are that some plants need to find a partner to sexually reproduce and that many plants require liquid water to be present so that sperm can swim.
The life cycles of plants are a bit more complicated than those of animals. In animals, haploid cells called gametes (sperm and eggs) are usually small and inconspicuous. In plants, however, haploid cells can literally have a life of their own.
Plant life cycles involve an alternation of generations: The complete cycle includes two different generations called sporophytes and gametophytes. Here’s a breakdown of the cycle:
Meiosis in a parent plant, called a sporophyte, results in the production of spores that are haploid, meaning they have half the genetic information of the parent plant.
The spores begin to grow by mitosis, developing into multicellular haploid organisms called gametophytes.
The gametophyte step of the plant life cycle is a fundamental difference between plants and animals. In animals, no development occurs until a sperm and an egg combine to produce a new organism. In plants, there’s a little break between meiosis and the production of sperm and eggs. During that break, a separate little haploid plant grows.
Gametophytes produce gametes by mitosis.
In animals, meiosis produces sperm and egg, but in plants, meiosis occurs to produce the gametophyte. The gametophyte is already haploid, so it produces sperm and egg by mitosis.
The gametes merge, producing cells called zygotes that contain the same number of chromosomes as the parent plant — that is, the zygotes are diploid.
Zygotes divide by mitosis and develop into sporophytes so the life cycle can begin again.
The plants you see when you go for a walk in the woods may be sporophytes or gametophytes:
Mosses that grow on trees and on the forest floor are gametophytes. If you see little structures like flagpoles sticking off the moss, you’re looking at a sporophyte. The little sporophytes grow like flags off the tops of the gametophytes. Inside the little flags, called capsules, meiosis is occurring to produce spores.
Ferns you can see are sporophytes. If you look on the back of a fern’s leaves, you can find little brown structures that seem dusty to the touch. These structures are where spores are being made, and the dust that comes off is the spores. Fern gametophytes are very tiny — about as big as the fingernail on your pinkie — making them very tough to find in the wild.
The conifers you see in a forest are sporophytes. The gametophyte generation in conifers is very small and contained within their cones.
Flowering plants that are visible to the eye are also sporophytes. In flowering plants, the gametophyte generation is very small and contained within the flowers.