What are Gymnosperms?

Gymnosperms, which translates to "naked seeds," are a diverse group of seed plants. The angiosperms are a sister group to one group of gymnosperms (the Gnetales), making the gymnosperms a paraphyletic group, according to the "anthophyte" hypothesis. The term "paraphyletic group" refers to a group that does not include all descendants of a single common ancestor. The "netifer" theory, on the other hand, indicates that Gnetophytes are sister to conifers, making gymnosperms monophyletic and sister to angiosperms. Further, molecular and anatomical research may shed light on these connections.

Characteristics of Gymnosperms

Naked seeds, separate female and male gametophytes, pollen cones and ovulate cones, pollination by wind and insects, and tracheid are all characteristics of gymnosperms (which transport water and solutes in the vascular system).

Gymnosperm seeds are not enclosed in an ovary; instead, modified leaves called sporophylls protect them only partially. The word strobilus (plural = strobili) refers to a dense cluster of sporophylls around a central stalk, as seen in pine cones. When seeds mature, they are encased in sporophyte tissues. The integument is a layer of sporophyte tissue that covers the megasporangium and, later, the embryo.

Evolution of Gymnosperms

In the Mesozoic period, Gymnosperms were the most common phylum. They've evolved to live in areas where freshwater is scarce for most of the year, or in bogs with nitrogen-deficient soil. As a result, they remain the dominant phylum in the coniferous biome, or taiga, where evergreen conifers benefit from the cold and dry climate. During the winter, evergreen conifers maintain low levels of photosynthesis and can take advantage of the first sunny days of spring. Conifers are more vulnerable to leaf infestations than deciduous trees because most conifers do not lose their leaves all at once.

Classification of Gymnosperms

Gymnosperms are divided into four phyla today. Coniferophyta, Cycadophyta, and Ginkgophyta have similar seed development patterns as well as secondary cambium production(cells that generate the vascular system of the trunk or stem and are partially specialized for water transportation). The three phyla, on the other hand, are not phylogenetically related. Since they develop true xylem tissue with vessels as well as the tracheid found in the rest of the gymnosperms, Gnetophyta is considered the closest group to angiosperms. Likely, the vessel components in the two classes evolved independently.

Gymnosperms are classified into four different classes:

Cycadales

These are the upright, unbranched plants that look like palms. The tropics and subtropics are where they are most commonly found. These plants have pinnately compound leaves that form a crown. Taproots and coralloid are their roots.

Ginkgoales

The large size and fan-shaped leaves of these plants distinguish them. The only living species in the group is Ginkgo biloba. All of the others have died out. G. biloba plant emits an unpleasant odor.

Coniferales

These are the most well-known gymnosperm species. They are evergreen trees that look like cones. As a result, they don't shed in the winter. Their leaves resemble needles or scales. These plants have winged seeds that are produced in female cones.

Gnetales

Angiosperms are a small group of plants that have advanced characteristics. Their flowery, soft-coated leaves are their most distinguishing feature. Their ovules are bare, just like the others, but they grow on a flower-like shoot.

The Life Cycle of Gymnosperms

A gymnosperm's life cycle involves generational alternation, with a dominant sporophyte containing reduced male and female gametophytes. Gymnosperms are all heterosporous plants. Cones or strobili are the male and female reproductive organs. Monoecious (“one home” or bisexual) plants grow male and female sporangia on the same plant, while dioecious (“two homes” or unisexual) plants produce male and female sporangia on different plants. The life cycle of a conifer will serve as a model for gymnosperm reproduction.

Pine trees are conifers (coniferous = cone-bearing), and each mature sporophyte contains both male and female sporophylls. As a result, they are monoecious. Pines, like all gymnosperms, are heterosporous, producing two types of spores (male microspores and female megaspores). Male and female spores grow in separate strobili, with male cones being smaller and female cones being larger. Microsporocytes undergo meiosis in male cones or staminate cones, and the haploid microspores that result give rise to male gametophytes or "pollen grains" by mitosis.

Each pollen grain is made up of a few haploid cells enclosed in a tough sporopollenin-reinforced wall. Significant quantities of yellow pollen are released in the spring and carried by the wind. A female cone will attract some gametophytes. The initiation of pollen tube development is referred to as pollination. The pollen tube grows slowly, and the pollen grain's generative cell undergoes mitosis to produce two haploid sperm or generative nuclei. Each of the haploid sperm nuclei will join with the haploid nucleus of an egg cell during fertilization.

Two ovules per scale are found in female cones, also known as ovulate cones. Each ovule has a small passage that opens near the sporophyll's base. The micropyle is the passage through which a pollen tube can expand later. In each ovule, one megaspore mother cell, or megasporocyte, goes through meiosis. Three of the four cells die, leaving only one to grow into a female multicellular gametophyte that encircles archegonia (an archegonium is a reproductive organ that contains a single large egg). A sticky pollination drop traps windblown pollen grains near the micropyle opening as the female gametophyte develops.

A pollen tube forms and expands toward the gametophyte as it develops. When the egg matures, one of the generative or sperm nuclei from the pollen tube will join the egg and fuse with the egg nucleus. The embryo is wrapped in a seed coat of tissue from the parent plant, which is formed when the diploid egg is fertilized. Although multiple eggs can be produced and even fertilized, each ovule normally only has one surviving embryo. In pine trees, fertilization and seed growth take a long time: it may take up to two years after pollination. The seed is composed of three generations of tissues: the seed coat that emerges from the sporophyte tissue, the gametophyte tissue that will provide nutrients, and the embryo itself.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for 

• Bachelors in Biology
• Bachelors in Botany
• Masters in Botany