Characteristics of Gymnosperms
Gymnosperms are seed plants that have evolved cones to carry their reproductive structures.
Discuss the type of seeds produced by gymnosperms
- Gymnosperms produce both male and female cones, each making the gametes needed for fertilization; this makes them heterosporous.
- Megaspores made in cones develop into the female gametophytes inside the ovules of gymnosperms, while pollen grains develop from cones that produce microspores.
- Conifer sperm do not have flagella but rather move by way of a pollen tube once in contact with the ovule.
- ovule: the structure in a plant that develops into a seed after fertilization; the megasporangium of a seed plant with its enclosing integuments
- sporophyll: the equivalent to a leaf in ferns and mosses that bears the sporangia
- heterosporous: producing both male and female gametophytes
Characteristics of Gymnosperms
Gymnosperms are seed plants adapted to life on land; thus, they are autotrophic, photosynthetic organisms that tend to conserve water. They have a vascular system (used for the transportation of water and nutrients) that includes roots, xylem, and phloem. The name gymnosperm means “naked seed,” which is the major distinguishing factor between gymnosperms and angiosperms, the two distinct subgroups of seed plants. This term comes from the fact that the ovules and seeds of gymnosperms develop on the scales of cones rather than in enclosed chambers called ovaries.
Gymnosperms are older than angiosperms on the evolutionary scale. They are found far earlier in the fossil record than angiosperms. As will be discussed in subsequent sections, the various environmental adaptations gymnosperms have represent a step on the path to the most successful (diversity-wise) clade (monophyletic branch).
Gymnosperm Reproduction and Seeds
Gymnosperms are sporophytes (a plant with two copies of its genetic material, capable of producing spores ). Their sporangia (receptacle in which sexual spores are formed) are found on sporophylls, plated scale-like structures that together make up cones. The female gametophyte develops from the haploid (meaning one set of genetic material) spores that are contained within the sporangia. Like all seed plants, gymnosperms are heterosporous: both sexes of gametophytes develop from different types of spores produced by separate cones. One type of cone is the small pollen cone, which produces microspores that subsequently develop into pollen grains. The other type of cones, the larger “ovulate” cones, make megaspores that develop into female gametophytes called ovules. Incredibly, this whole sexual process can take three years: from the production of the two sexes of gametophytes, to bringing the gametophytes together in the process of pollination, and finally to forming mature seeds from fertilized ovules. After this process is completed, the individual sporophylls separate (the cone breaks apart) and float in the wind to a habitable place. This is concluded with germination and the formation of a seedling. Conifers have sperm that do not have flagella, but instead are conveyed to the egg via a pollen tube. It is important to note that the seeds of gymnosperms are not enclosed in their final state upon the cone.
Life Cycle of a Conifer
Conifers are monoecious plants that produce both male and female cones, each making the necessary gametes used for fertilization.
Describe the life cycle of a gymnosperm
- Male cones give rise to microspores, which produce pollen grains, while female cones give rise to megaspores, which produce ovules.
- The pollen tube develops from the pollen grain to initiate fertilization; the pollen grain divides into two sperm cells by mitosis; one of the sperm cells unites with the egg cell during fertilization.
- Once the ovule is fertilized, a diploid sporophyte is produced, which gives rise to the embryo enclosed in a seed coat of tissue from the parent plant.
- Fetilization and seed development can take years; the seed that is formed is made up of three tissues: the seed coat, the gametophyte, and the embryo.
- megaspore: the larger spore of a heterosporous plant, typically producing a female gametophyte
- microspore: a small spore, as contrasted to the larger megaspore, which develops into male gametophytes
- monoecious: having the male (stamen) and female (carpel) reproductive organs on the same plant rather than on separate plants
Life Cycle of a Conifer
Pine trees are conifers (cone bearing) and carry both male and female sporophylls on the same mature sporophyte. Therefore, they are monoecious plants. Like all gymnosperms, pines are heterosporous, generating two different types of spores: male microspores and female megaspores. In the male cones (staminate cones), the microsporocytes give rise to pollen grains by meiosis. In the spring, large amounts of yellow pollen are released and carried by the wind. Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube growth. The pollen tube develops slowly as the generative cell in the pollen grain divides into two haploid sperm cells by mitosis. At fertilization, one of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of an egg cell.
Female cones (ovulate cones) contain two ovules per scale. One megaspore mother cell (megasporocyte) undergoes meiosis in each ovule. Three of the four cells break down leaving only a single surviving cell which will develop into a female multicellular gametophyte. It encloses archegonia (an archegonium is a reproductive organ that contains a single large egg). Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte that will provide nutrients, and the embryo itself.
In the life cycle of a conifer, the sporophyte (2n) phase is the longest phase. The gametophytes (1n), microspores and megaspores, are reduced in size. This phase may take more than one year between pollination and fertilization while the pollen tube grows towards the megasporocyte (2n), which undergoes meiosis into megaspores. The megaspores will mature into eggs (1n).
Diversity of Gymnosperms
Gymnosperms are a diverse group of plants the protect their seeds with cones and do not produce flowers or fruits.
Give examples showing the diversity of gymnosperms
- Gymnosperms consist of four main phyla: the Coniferophyta, Cycadophyta, Gingkophyta and Gnetophyta.
- Conifers are the dominant plant of the gymnosperms, having needle-like leaves and living in areas where the weather is cold and dry.
- Cycads live in warm climates, have large, compound leaves, and are unusual in that they are pollinated by beetles rather than wind.
- Gingko biloba is the only remaining species of the Gingkophyta and is usually resistant to pollution.
- Gnetophytes are the gymnosperms believed to be most closely related to the angiosperms because of the presence of vessel elements within their stems.
- tracheid: elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts
- angiosperm: a plant whose ovules are enclosed in an ovary
- conifer: a plant belonging to the conifers; a cone-bearing seed plant with vascular tissue, usually a tree
Diversity of Gymnosperms
Modern gymnosperms are classified into four phyla. The first three (the Coniferophyta, Cycadophyta, and Gingkophyta) are similar in their production of secondary cambium (cells that generate the vascular system of the trunk or stem and are partially specialized for water transportation) and their pattern of seed development. However, these three phyla are not closely related phylogenetically to each other. The fourth phylum (the Gnetophyta) are considered the closest group to angiosperms because they produce true xylem tissue.
Conifers are the dominant phylum of gymnosperms, with the most variety of species. They are typically tall trees that usually bear scale-like or needle-like leaves. Water evaporation from leaves is reduced by their thin shape and the thick cuticle. Snow slides easily off needle-shaped leaves, keeping the load light and decreasing breaking of branches. Adaptations to cold and dry weather explain the predominance of conifers at high altitudes and in cold climates. Conifers include familiar evergreen trees such as pines, spruces, firs, cedars, sequoias, and yews. A few species are deciduous, losing their leaves in fall. The European larch and the tamarack are examples of deciduous conifers. Many coniferous trees are harvested for paper pulp and timber. The wood of conifers is more primitive than the wood of angiosperms; it contains tracheids, but no vessel elements, and is, therefore, referred to as “soft wood.”
Cycads thrive in mild climates. They are often mistaken for palms because of the shape of their large, compound leaves. Cycads bear large cones and may be pollinated by beetles rather than wind, which is unusual for a gymnosperm (). They dominated the landscape during the age of dinosaurs in the Mesozoic, but only a hundred or so species persisted to modern times. Cycads face possible extinction; several species are protected through international conventions. Because of their attractive shape, they are often used as ornamental plants in gardens in the tropics and subtropics.
The single surviving species of the gingkophytes group is the Gingko biloba. Its fan-shaped leaves, unique among seed plants because they feature a dichotomous venation pattern, turn yellow in autumn and fall from the tree. For centuries, G. biloba was cultivated by Chinese Buddhist monks in monasteries, which ensured its preservation. It is planted in public spaces because it is unusually resistant to pollution. Male and female organs are produced on separate plants. Typically, gardeners plant only male trees because the seeds produced by the female plant have an off-putting smell of rancid butter.
Gnetophytes are the closest relative to modern angiosperms and include three dissimilar genera of plants: Ephedra, Gnetum, and Welwitschia. Like angiosperms, they have broad leaves. In tropical and subtropical zones, gnetophytes are vines or small shrubs. Ephedra occurs in dry areas of the West Coast of the United States and Mexico. Ephedra’s small, scale-like leaves are the source of the compound ephedrine, which is used in medicine as a potent decongestant. Because ephedrine is similar to amphetamines, both in chemical structure and neurological effects, its use is restricted to prescription drugs. Like angiosperms, but unlike other gymnosperms, all gnetophytes possess vessel elements in their xylem.