How does a sporophyte become a gametophyte

These spores are haploid cells that grow into haploid gametophytes. Megaspores grow into female gametophytes, and microspores grow into male gametophytes. Meiosis occurs in the sporangium of a sporophyte and results in haploid spores. These spores contain one cell that can become another new plant without mating.

Sporophytes have evolved in vascular plants to become larger, more dominant and longer-lived compared to gametophytes. Gametophytes are haploid plants that use mitosis to make haploid gametes. These gametes are female in the form of an ovum egg or male in the form of sperm. Gametophytes contain the archegonium, or female sex organ, or they contain the antheridium, or male sex organ. The sperm and egg unite in the archegonium to produce a diploid zygote cell. That zygote becomes a sporophyte.

Vascular plant gametophytes tend to be much smaller than sporophytes, sometimes even only a few cells in size.

A pollen grain represents an example of a male gametophyte in vascular plants. Vascular and non-vascular plants display interesting differences between their sporophytes and gametophytes. Vascular plants do not require as much water to thrive, and they display their large, long-lived sporophyte phase as the actual plant.

Gymnosperms such as conifers contain a bit of female gametophyte tissue in their cones, such as pine nuts. Those nuts contain the embryonic diploid sporophyte. The male conifer gametophyte exists as pollen, which is wind-dispersed. For flowering plants such as fruit trees and flowers, female gametophytes contain a few cells and reside inside the ovary of the flower; the male exists as pollen. The small gametophytes of vascular plants only live for a season.

Vascular plants that make two kinds of spores and gametophytes are called heterosporic. Most flowers have a mutualistic pollinator, with the distinctive features of flowers reflecting the nature of the pollination agent. The relationship between pollinator and flower characteristics is one of the great examples of coevolution.

Following fertilization of the egg, the ovule grows into a seed. The surrounding tissues of the ovary thicken, developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range. Like flowers, fruit can vary tremendously in appearance, size, smell, and taste. Tomatoes, green peppers, corn, and avocados are all examples of fruits.

Along with pollen and seeds, fruits also act as agents of dispersal. Some may be carried away by the wind. Many attract animals that will eat the fruit and pass the seeds through their digestive systems, then deposit the seeds in another location. Cockleburs are covered with stiff, hooked spines that can hook into fur or clothing and hitch a ride on an animal for long distances. The cockleburs that clung to the velvet trousers of an enterprising Swiss hiker, George de Mestral, inspired his invention of the loop and hook fastener he named Velcro.

All living organisms display patterns of relationships derived from their evolutionary history. Phylogeny is the science that describes the relative connections between organisms, in terms of ancestral and descendant species. Phylogenetic trees, such as the plant evolutionary history shown in Figure 5, are tree-like branching diagrams that depict these relationships. Species are found at the tips of the branches. Each branching point, called a node, is the point at which a single taxonomic group taxon , such as a species, separates into two or more species.

Figure 5. This phylogenetic tree shows the evolutionary relationships of plants. Traditional methods involve comparison of homologous anatomical structures and embryonic development, assuming that closely related organisms share anatomical features that emerge during embryo development. Some traits that disappear in the adult are present in the embryo; for example, an early human embryo has a postanal tail, as do all members of the Phylum Chordata.

The study of fossil records shows the intermediate stages that link an ancestral form to its descendants. However, many of the approaches to classification based on the fossil record alone are imprecise and lend themselves to multiple interpretations. As the tools of molecular biology and computational analysis have been developed and perfected in recent years, a new generation of tree-building methods has taken shape. The key assumption is that genes for essential proteins or RNA structures, such as the ribosomal RNAs, are inherently conserved because mutations changes in the DNA sequence could possibly compromise the survival of the organism.

DNA from minute samples of living organisms or fossils can be amplified by polymerase chain reaction PCR and sequenced, targeting the regions of the genome that are most likely to be conserved between species.

Once the sequences of interest are obtained, they are compared with existing sequences in databases such as GenBank, which is maintained by The National Center for Biotechnology Information. A number of computational tools are available to align and analyze sequences. Sophisticated computer analysis programs determine the percentage of sequence identity or homology.

Sequence homology can be used to estimate the evolutionary distance between two DNA sequences and reflect the time elapsed since the genes separated from a common ancestor. Molecular analysis has revolutionized phylogenetic trees. In some cases, prior results from morphological studies have been confirmed: for example, confirming Amborella trichopoda as the most primitive angiosperm known.

However, some groups and relationships have been rearranged as a result of DNA analysis. Seed plants appeared about one million years ago, during the Carboniferous period.

Two major innovations—seed and pollen—allowed seed plants to reproduce in the absence of water. The gametophytes of seed plants shrank, while the sporophytes became prominent structures and the diploid stage became the longest phase of the lifecycle. Gymnosperms became the dominant group during the Triassic. In these, pollen grains and seeds protect against desiccation. The seed, unlike a spore, is a diploid embryo surrounded by storage tissue and protective layers.

It is equipped to delay germination until growth conditions are optimal. Angiosperms bear both flowers and fruit. The structures protect the gametes and the embryo during its development. Angiosperms appeared during the Mesozoic era and have become the dominant plant life in terrestrial habitats. Improve this page Learn More. Skip to main content. Module 7: Plant Diversity. Search for:. In Summary: Evolution of Seed Plants Seed plants appeared about one million years ago, during the Carboniferous period.

Try It. The gametophytes are very small and cannot exist independent of the parent plant. The reproductive structures of the sporophyte cones in gymnosperms and flowers in angiosperms , produce two different kinds of haploid spores: microspores male and megaspores female. This phenomenon of sexually differentiated spores is called heterospory. These spores give rise to similarly sexually differentiated gametophytes, which in turn produce gametes.

Fertilization occurs when a male and female gamete join to form a zygote. The resulting embryo, encased in a seed coating, will eventually become a new sporophyte. SparkTeach Teacher's Handbook. Summary Alternation of Generations. Bryophyte Generations Bryophytes are nonvascularized plants that are still dependent on a moist environment for survival see Plant Classification, Bryophytes.