Gymnosperms and angiosperms are both able to produce

Updated June 26, 2019

By Mary Dowd

Life as we know it would not exist without plants to convert sunlight and inorganic compounds into food energy. In Kingdom Plantae, plant species are classified according to their method of reproduction.

One group is the "seed plants," which can be divided into two subgroups called angiosperms and gymnosperms.

Angiosperm derives from the Greek words for "vessel" and "seed." Angiosperms include vascular land plants and hardwood trees with flowers and fruit. They reproduce by making seeds that are enclosed in an ovary.

Gymnosperm derives from the Greek words for "naked seeds." Gymnosperms include vascular land plants and softwood trees that do not have flowers and fruit. They are cone-bearing and reproduce by making naked seeds on cone scales or leaves.

Plant life evolved millions of years ago from primitive algae in the sea. Nonvascular mosses, liverworts and hornworts then arrived on the scene. These types of living species reproduce by fragmentation or spores. Next came seedless vascular plants like ferns and horsetails.

Plants with a vascular system were stronger and able to grow taller. Gymnosperms, like conifers and ginko biloba, appeared during the Paleozoic Era and reproduced by dispersing “naked seeds” not imbedded in flowers or fruit.

Angiosperms evolved later during the Mesozoic Era. Angiosperms adapted to a challenging terrestrial ecosystem by developing a complex vascular system, flowers and fruit. They reproduced by seed and spread quickly on land.

Gymnosperms and angiosperms are more highly evolved than nonvascular plants. Both are vascular plants with vascular tissue that live on land and reproduce by making seeds.

They are also classified as eukaryotes, meaning they have a membrane-bound nucleus.

Only angiosperms are known as flowering plants. Many have beautiful petals, fragrant blossoms and fruit that contains dozens of seeds. Angiosperms typically drop their leaves when the seasons change and chlorophyll production ceases.

By contrast, gymnosperms such as pine trees produce bare, uncovered seeds, usually in pine cones. Most gymnosperms have green, needle-like leaf structures; angiosperm leaves are flat_._ Angiosperm leaves are seasonal in their life cycle while gymnosperms are generally evergreen.

The flowers of angiosperms have male and female reproductive parts. Stamens are male sex structures that make pollen on their anthers.

Pollination occurs when pollen grains from the anther reach the pistil, which is the flower’s female structure. A pollen tube in a structure called the style helps the generative cell in pollen reach the ovarian embryo sac.

The generative cell in pollen splits into two sperm cells. One fertilizes the egg, and the other one helps make endosperm through a process known as double fertilization. Fertilized eggs mature into seeds protected inside fruit.

Sporophytes in gymnosperms make male and female gametophytes. For instance, male cones have male gametophytes (pollen), and they are smaller than cones with female gametophytes.

Wind carries pollen from male to female cones. The fertilized female gametophyte produces a seed on a scale inside the cone.

Pollination methods of angiosperms differ somewhat from those of gymnosperms.

Angiosperms rely on bird, bees and other pollinators, as well as abiotic factors such as wind and water. Gymnosperms rely solely on the wind to carry pollen between male and female reproductive parts.

Unlike angiosperms, some species of gymnosperms have been around since the days of the dinosaur. For example, cycads (in the division known as Cycadophyta) look like palm trees, but they are actually close relatives of Coniferophyta (conifers) and Ginkgophyta (the division that contains Ginkgo biloba).

Gnetophyta, like the Welwitschia mirabilis desert plant, have existed for at least 145 million years based on fossil evidence. The Welwitschia can live up to 1,500 years. DNA shows that it is closely related to conifers and other gymnosperms, although the plant also has flower parts. It has been speculated that angiosperms may have evolved from gnetophytes.

The plant life cycle alternates between haploid and diploid generations. Embryonic development is seen only in the diploid generation. The embryo, however, is produced by the fusion of gametes, which are formed only by the haploid generation. So understanding the relationship between the two generations is important in the study of plant development.

Unlike animals(see Chapter 2), plants have multicellular haploid and multicellular diploid stages in their life cycle. Gametes develop in the multicellular haploid gametophyte (from the Greek phyton, “plant”). Fertilization gives rise to a multicellular diploid sporophyte, which produces haploid spores via meiosis. This type of life cycle is called a haplodiplontic life cycle (Figure 20.1). It differs from our own diplontic life cycle, in which only the gametes are in the haploid state. In haplodiplontic life cycles, gametes are not the direct result of a meiotic division. Diploid sporophyte cells undergo meiosis to produce haploid spores. Each spore goes through mitotic divisions to yield a multicellular, haploid gametophyte. Mitotic divisions within the gametophyte are required to produce the gametes. The diploid sporophyte results from the fusion of two gametes. Among the Plantae, the gametophytes and sporophytes of a species have distinct morphologies (in some algae they look alike). How a single genome can be used to create two unique morphologies is an intriguing puzzle.

All plants alternate generations. There is an evolutionary trend from sporophytes that are nutritionally dependent on autotrophic (self-feeding) gametophytes to the opposite‐gametophytes that are dependent on autotrophic sporophytes. This trend is exemplified by comparing the life cycles of a moss, a fern, and an angiosperm (see Figures 20.2– 20.4). (Gymnosperm life cycles bear many similarities to those of angiosperms; the distinctions will be explored in the context of angiosperm development.)

The “leafy” moss you walk on in the woods is the gametophyte generation of that plant (Figure 20.2). Mosses are heterosporous, which means they make two distinct types of spores; these develop into male and female gametophytes. Male gametophytes develop reproductive structures called antheridia (singular, antheridium) that produce sperm by mitosis. Female gametophytes develop archegonia (singular, archegonium) that produce eggs by mitosis. Sperm travel to a neighboring plant via a water droplet, are chemically attracted to the entrance of the archegonium, and fertilization results.* The embryonic sporophyte develops within the archegonium, and the mature sporophyte stays attached to the gametophyte. The sporophyte is not photosynthetic. Thus both the embryo and the mature sporophyte are nourished by the gametophyte. Meiosis within the capsule of the sporophyte yields haploid spores that are released and eventually germinate to form a male or female gametophyte.

Ferns follow a pattern of development similar to that of mosses, although most (but not all) ferns are homosporous. That is, the sporophyte produces only one type of spore within a structure called the sporangium (Figure 20.3). One gametophyte can produce both male and female sex organs. The greatest contrast between the mosses and the ferns is that both the gametophyte and the sporophyte of the fern photosynthesize and are thus autotrophic; the shift to a dominant sporophyte generation is taking place.†

At first glance, angiosperms may appear to have a diplontic life cycle because the gametophyte generation has been reduced to just a few cells (Figure 20.4). However, mitotic division still follows meiosis in the sporophyte, resulting in a multicellular gametophyte, which produces eggs or sperm. All of this takes place in the the organ that characterizes the angiosperms: the flower. Male and female gametophytes have distinct morphologies (i.e., angiosperms are heterosporous), but the gametes they produce no longer rely on water for fertilization. Rather, wind or members of the animal kingdom deliver the male gametophyte—pollen—to the female gametophyte. Another evolutionary innovation is the production of a seed coat, which adds an extra layer of protection around the embryo. The seed coat is also found in the gymnosperms. A further protective layer, the fruit, is unique to the angiosperms and aids in the dispersal of the enclosed embryos by wind or animals.

The remainder of this chapter provides a detailed exploration of angiosperm development from fertilization to senescence. Keep in mind that the basic haplodiplontic life cycle seen in the mosses and ferns is also found in the angiosperms, continuing the trend toward increased nourishment and protection of the embryo.

*

Have you ever wondered why there are no moss trees? Aside from the fact that the gametophytes of mosses (and other plants) do not have the necessary structural support and transport systems to attain tree height, it would be very difficult for a sperm to swim up a tree!

It is possible to have tree ferns, for two reasons. First, the gametophyte develops on the ground, where water can facilitate fertilization. Secondly, unlike mosses, the fern sporophyte has vascular tissue, which provides the support and transport system necessary to achieve substantial height.