Gymnosperms

What are gymnosperms? How do they differ from angiosperms?

Let's start by looking at the definition of gymnosperms.

Recall that Vascular Plants are those with specialized tissues that transport water and nutrients to various parts of the plant. Vascular plants can be divided into higher and lower vascular plants: higher vascular plants produce seeds, while lower vascular plants do not.

The Difference between Angiosperms and Gymnosperms

Higher vascular plants can be further divided into Angiosperms and gymnosperms.

Angiosperms and gymnosperms primarily differ in how their seeds are developed: in angiosperms, the seed is contained in an ovary, whereas in gymnosperms, the seed lies exposed on cones or modified leaves (Fig. 1).

Angiosperms also differ from gymnosperms in other ways. Angiosperms produce flowers, and for this reason, they are also called flowering plants. Their flowers are pollinated by water, wind, and Animals. After Pollination and fertilization, their flowers--which contain the ovary--develop into seed-bearing fruits. Their fruits aid in the dispersal of seeds.

Gymnosperms, on the other hand, do not produce flowers or fruits.

Examples of Angiosperms and Gymnosperms

Angiosperms are the largest group of plants, with over 300,000 species, including dandelions, oranges, almonds, and roses. On the other hand, gymnosperms have only around 1,000 extant species. Some familiar examples of gymnosperms include pines, cypresses, sago palms, and ginkgos.

In the following sections, we will delve deeper into the evolution, characteristics, and classification of gymnosperms.

What is the Evolutionary History of Gymnosperms?

The earliest group of gymnosperms are seed ferns called Pteridospermophyta, which are now extinct and are only known from fossil evidence (note that non-extinct ferns reproduce by spores, not seeds).

These seed ferns bore seeds directly on their leaves or branches without being contained in specialized structures, like cones or fruits. Then, later in evolutionary history, the exposed ovules and cones characteristic of gymnosperms emerged.

For around 200 million years prior to the emergence of flowering plants, gymnosperms dominated the world. However, extreme climatic changes led to the Extinction of many gymnosperm species. This accounts for the relatively low diversity among extant gymnosperm species.

What are the Characteristics of Gymnosperms?

Now, let's explore the characteristics of gymnosperms.

As vascular plants, gymnosperms have two types of vascular tissues: Xylem and Phloem.

Except for gnetales (which will be described later), gymnosperms lack vessel elements and only have tracheids.

The bracts of a gymnosperm cone are called sporophylls. Sporophylls are arranged around a central stalk. This arrangement is called strobilus (plural: strobili). When mature, the seeds of some species are enclosed in sporophyte tissue called integument.

Male pollen cones called microstrobili have small leaves called microsporophylls. Pollen sacs called microsporangia are found on the lower surface of microsporophylls. On the other hand, female ovulate cones called megastrobili have many scales called megasporophylls, each containing megasporangia.

Male and female sporangia can be found in the same plant (called monoecious) or on separate plants (called dioecious).

Figure 2 below shows the basic structure of a male and female pine cone.

The life cycle and reproduction of gymnosperms

An alternation of generations characterizes the life cycle of gymnosperms: haploid and diploid generations alternately produce each other (Fig. 3).

Diploid means having two sets of Chromosomes (one from each parent), while haploid means having a single set of Chromosomes. The diploid nonsexual phase of a plant is called sporophyte, while the haploid sexual phase of the plant is called a gametophyte.

The sporophyte generation of the gymnosperm is more dominant than its gametophyte generation.

Diploid cells within the microsporangia undergo Meiosis to produce haploid microspores. The microspore eventually germinates within the microsporangium, forming the male gametophyte (pollen grain). At this stage, the plant sheds pollen, which is then transported by wind or insects.

Similarly, a diploid cell within each megasporangium undergoes Meiosis, resulting in four haploid megaspores, three of which degenerate, while the remaining one undergoes Mitosis to form the female gametophyte. This prepares the ovule for fertilization.

Before fertilization, the pollen must first be transported to the female gametophyte in a process called Pollination.

In the female megasporangium of many gymnosperms, a small pore exudes a sticky "pollination droplet" that traps pollen grains. The droplet is reabsorbed into the megasporangium for fertilization. In other species, pollen grains settle and grow on the surface of the megasporangium.

Once a pollen grain germinates, a pollen tube emerges and extends through the megasporangium toward the archegonium, an egg-containing structure. As the sperm (contained within the pollen grain) and egg mature, the sperm's nucleus undergoes further division to produce two sperm cells. When the nuclei of the sperm come into contact with the egg cell, one nucleus dies while the other nucleus fuses with the egg cell, forming a diploid zygote.

When the zygote undergoes Mitosis, a new sporophyte generation (in the form of a seed embryo) is formed. The fertilization process of gymnosperms is illustrated in Figure 4 below.

What is the classification of gymnosperms?

Four groups comprise gymnosperms: cycads, ginkgo, conifers, and gnetales (Fig. 5).

Cycads

Cycads have existed for 280 million years. They used to be found all over the planet but, over time, have become limited to the tropics and subtropics. Much of the remaining cycads we know today are survivors of a once successful plant group.

There are around 300 extant species of cycads. A distinguishing feature of cycads is that they have a single unbranched trunk with leaves clustered together at the top of the plant. However, this same characteristic makes it appear similar to palms. Such is reflected in the name given to a cycad, sago palm, which is also the plant shown in Figure 5.

Cycads are dioecious, meaning male plants produce pollen cones, while female plants produce seed cones. The cones of cycads are usually large, with multiple sporophylls aggregated into cones. In pollen and seed cones, the sporophylls are attached directly to the cone axis or column with no other leaves or bracts attached.

Ginkgo

Ginkgo biloba is the only nonextinct species of the order Ginkgoales, so some refer to it as a "living fossil."

It has characteristics of both cycads and conifers, so it is often considered an intermediate between the two groups.

Its distinguishing characteristics include its fan-shaped leaves. Unlike the cycads, its crown is broad, and it is heavily branched. Ginkgos are also dioecious: the male pollen cones have long stalks with pollen-producing organs attached to them, while the female cones consist of a stalk and two terminal ovules.

Conifers

With over 600 species, including pines, spruces, and cedars, conifers are the dominant group among gymnosperms, as they make up 39% of the world's forests.

Conifers typically have needle-shaped evergreen leaves. Their thin shape and thick cuticle help reduce evaporation and prevent snow from piling on branches. Such adaptations explain their abundance in colder and drier climates.

Several conifers, including larches, are deciduous. So, unlike evergreen conifers, they lose their leaves during the fall.

Conifers' seeds are typically attached to the scales of a cone. An exception to this is the Taxus family, whose seeds are not found on scales, but instead at the end of short branches. An outgrowth at the base of the taxus seed eventually becomes a fleshy red aril (seed covering).

Gnetales

Gnetales are made up of three subgroups: Ephedraceae, Gnetaceae, and Welwitschiaceae. The three subgroups are so different from each other that you might not think they are related at first glance.

Ephedraceae are typically found in dry climates. These are usually shrubs or vines, although a few are small trees. Their scale-like leaves are not photosynthetic, but their ribbed, round green stems are. Most species are dioecious. Some species have seeds enclosed in a brightly-colored fleshy integument, making them look like fruits.

Gnetaceae are tropical gymnosperms, most of which are woody vines. Their seeds are often enclosed in a brightly-colored false seed coat, making them look like berries. Their leaves are broad, flat and have netlike veins, making them superficially resemble angiosperms.

Welwitschiaceae consists of only one species (Welwitschia mirabilis) that grows in the Namib Desert. It produces only two massive leaves from a short woody stem. These leaves can grow up to 6 meters (or 19.7 feet) long!. Its cones develop from buds on a woody crown between the two leaves.

While they are vastly different, these three groups share some characteristics, including the presence of vessel elements (which distinguish them from other gymnosperms) and striated pollen.

Gnetales are also so different from other seed plants that much of their evolutionary relationship with other groups remains unclear. For example, some researchers place them close to angiosperms, while others associate them with conifers.