Classification of Gymnosperms | Botany Optional for UPSC PDF Download

Definition of Gymnosperms

Gymnosperms, a term coined by Theophrastus in 300 BC, refer to plants with unprotected seeds. They are a group of phanerogams that lack ovaries, reproducing through seeds rather than spores. Unlike angiosperms (flowering plants), gymnosperms do not enclose their seeds within mature ovaries or fruits. Instead, the seeds are exposed on the surface of megasporophylls, which are often arranged in cones.

Gymnosperms likely evolved about 300 million years ago from non-seed producing ancestors known as Progymnospermophyta. These ancestors had fern-like characteristics, serving as a bridge between pteridophytes (ferns and their allies) and angiosperms (flowering plants).

Characteristics of Gymnosperm

At present, there are approximately 790 species of living gymnosperms distributed across temperate, tropical, and arctic regions worldwide.
Key characteristics of gymnosperms include:

• Gymnosperms constitute a relatively small group of seed plants, with about 900 living species.
• They are ancient plants, once dominant on Earth approximately 200 million years ago during the Mesozoic era. However, they now thrive primarily in cold regions, where snow rather than rain is the main water source.
• Gymnosperms are typically perennial and woody, often forming bushes or trees. Some gymnosperms, such as Sequoia sempervirens (the tallest gymnosperm at 111.6 meters), can live for thousands of years.
• These plants lack true flowers. Instead, they have two types of sporophylls: microsporophylls and megasporophylls, which form separate cones (pollen cones and seed cones, respectively).
• Gymnosperms do not produce fruits; their seeds are exposed.
• Unlike angiosperms, gymnosperms lack distinct structures like ovaries, styles, and stigmas.
• Gymnosperms have orthotropous and sessile ovules, each surrounded by a three-layered integument.
• Female gametophytes contain archegonia.
• Pollination is direct, often achieved by wind (anemophily).
• Male gametophytes produce two male gametes (sperm), with one usually being functional.
• Instead of external water for transporting male gametes, gymnosperms use pollen tubes (siphonogamy).
• Gymnosperm seeds contain endosperm, a nutrient-rich tissue for the embryo's growth.
• Xylem in gymnosperms typically lacks vessels, although some gneophytes have them. Phloem lacks companion cells and sieve tubes, with sieve cells not arranged end to end in rows.
• Vascular tissues are organized into vascular bundles, similar to angiosperms. Open vascular bundles allow for secondary growth.

Classification of Gymnosperms

In older times gymnosperms were kept among angiosperms. It was Robert Brown (1827) who first of all recognised these plants due to presence of naked ovules and placed them in a distinct group called gymnosperms. Bentham and Hooker (1862-83) in their ‘Genera Planterum’ placed this group in between dicotyledonae and monocotyledonae. The classification of gymnosperms is quite controversial because several genera and a few orders like the cordiatales and cycadeoidales are known only in fossil state. Attempts have, however, been made from time to time to classify them.

Gifford and Foster (1989) raised the important groups to the rank of division

Characteristics of Cycadophyta

• Cycads are typically adapted to arid conditions.
• These plants have a low, palm-like growth habit.
• Their stem is short, unbranched, columnar, and covered with persistent leaf bases.
• Cycad leaves are compound with leaflets arranged in a terminal crown.
• Cycads exhibit slow growth rates and exceptionally long lifespans.
• They have a relatively large pith and broad cortex.
• Cycads possess a narrow zone of conducting tissue, with vascular bundles around the pith separated by medullary rays.
• Cycads are strictly dioecious, meaning male and female reproductive structures develop on separate plants, except for the female strobilus in Cycas.
• Cycad ovules are straight and usually sessile, featuring a pollen chamber for pollen reception.
• The male gametes in cycads are capable of movement.

Characteristics of Coniferopsida

• Most conifers are evergreen and may have branched stems, occasionally growing as shrubs.
• Conifer leaves can be needle-like or scale-like, often arranged spirally or in whorls, and are usually adapted to dry conditions.
• Their wood lacks vessels and consists mainly of long tracheids with bordered pits, often containing resin canals.
• Conifers can have monoecious or dioecious flowers. Female flowers are terminal or surrounded by supporting bracts.
• Male flowers have multiple stamens arranged in strobili, each with several pollen sacs and often with an appendage on the connective.
• Some conifers produce winged pollen grains.
• Female flowers are typically found in cones or catkins, except for certain conifer families.
• Each female flower comprises a sterile bract and a fertile scale, with ovules developing on the upper surface of ovuliferous scales.
• Female cones often become dry upon ripening.
• Conifer seeds are often winged and have a woody or leathery outer covering.
• The cotyledons in conifers emerge above the ground (epigeal) and may range from 2 to 16 in number.
• Polyembryony is common in conifers.
• Conifers produce non-motile sperm during fertilization.

Characteristics of Gnetophyta

• Gnetophytes encompass a variety of woody plants, including trees, shrubs, and some with unusual shapes.
• Their leaves can be simple, elliptical, strap-shaped, or reduced to tiny scales, typically arranged oppositely or in whorls.
• Vessels are present in the secondary wood of gnetophytes.
• Gnetophyte "flowers" are unisexual, often dioecious, and occasionally monoecious.
• These unisexual "flowers" are grouped into compound strobili or inflorescences.
• Male "flowers" have a perianth and contain antherophores with one to eight synangia.
• Each female "flower" contains a single erect ovule.
• The ovule's nucellus is surrounded by two or three envelopes.
• The micropyle of the ovule extends as a long bristle-like tube.
• During fertilization, the pollen tube contains two male nuclei.
• A single-celled primary suspensor is present in the embryo.
• Gnetophyte embryos typically have two cotyledons.

Affinities and Relationship of Gymnosperms

Gymnosperms occupy a place in between pteridophytes and angiosperms in the plant kingdom. Therefore, gymnosperms bear close affinities with the pteridophytes on the one hand and the angiosperms on the other. In many other characters they differ from both.
Affinities and relationship of gymnosperms with other groups of plants are as follows: Resemblances or Similarities with Pteridophytes:

• Both gymnosperms and pteridophytes share a common characteristic of having an independent, mature plant body known as the sporophyte, which is comprised of differentiated structures like roots, stems, and leaves.
• Both groups possess a well-developed system of vascular tissues within their sporophytes.
• In both gymnosperms and pteridophytes, the xylem lacks vessels, and the phloem lacks companion cells.
• The early leaves of both gymnosperms and pteridophytes exhibit a coiled or curled growth pattern known as circinate vernation.
• Both gymnosperms and some pteridophytes have leaves known as megaphylls.
• Gymnosperms, as well as a few pteridophytes like Selaginella, are characterized by heterospory, which means they produce both microspores and megaspores in microsporangia and megasporangia, respectively, located on microsporophylls and megasporophylls.
• In Cycas, the sporangia are arranged in clusters called sori, similar to how they are organized in pteridophytes.
• In both groups, the female reproductive structure is called an archegonium.
• The male gametes of Cycas and Ginkgo are mobile, resembling the motile sperm of pteridophytes.
• Both gymnosperms and pteridophytes retain the megaspore within the megasporangium.
• The gametophytes of both gymnosperms and pteridophytes are endosporic (developing within the spore wall) and undergo significant reduction in size.
• Before fertilization, the female prothallus (gametophyte) develops, and there is a stage of free nuclear division in both gymnosperms and pteridophytes.
• The germination of spores is premature in gymnosperms and heterosporous pteridophytes, meaning they start growing before reaching full maturity.
• After fertilization, both gymnosperms and pteridophytes develop a distinct embryo.
• Similar to pteridophytes, gymnosperms also display a pronounced alternation of generations, with a dominant and independent sporophytic phase and a progressively reduced and dependent gametophytic phase throughout their life cycle.

Difference between Gymnosperms and Pteridophytes

Similarities with Angiosperms

• The primary body of the plant is the sporophyte, consisting of differentiated structures such as roots, stems, and leaves.
• These plants can be trees or shrubs, with variations in growth patterns that can be erect or climbing.
• They possess a well-developed root system, which can exhibit various arrangements like diarch, triarch, tetrarch, or polyarch.
• The xylem in the roots is exarch in nature.
• The stem structure is eustelic, featuring vascular bundles that are conjoint, collateral, open, and endarch.
• Secondary growth occurs in these plants.
• Wood can either be monoxylic (single type of wood) or polyxylic (multiple types of wood).
• In some gymnosperms, like the Gnetales group, vessels and companion cells are also present, resembling the features found in angiosperms.
• These plants are heterosporous, producing reduced gametophytes.
• An ovule is formed when the nucellus is surrounded by integuments.
• Similar to gymnosperms, many angiosperms rely on wind pollination.
• The megaspore remains within the megasporangium and develops into the female gametophyte.
• Pollen grains grow into pollen tubes during pollination.
• In the majority of gymnosperms and angiosperms, male gametes are non-motile.
• Fertilization is referred to as siphonogamous, involving the transfer of male gametes through pollen tubes.
• A suspensor structure is formed during the embryo's development.
• Endosperm formation is part of the process.
• Seeds are produced from ovules.
• Similar to gymnosperms, several angiosperms also exhibit polyembryony.
• The process of embryogeny is endoscopic in nature.
• The life cycle of both groups exhibits similarities.

Difference between Gymnosperms and Angiosperms

Economic Importance of Gymnosperms

• Ornamental Value:
  • Several gymnosperms, such as Cycas, Araucaria, and Thuja, are cultivated for their aesthetic appeal as ornamental plants.
• Food Value:
  • Sago: Starch extracted from the stem pith and cortex of plants like Cycas revoluta and Cycas rumphii is used to produce "sago" starch.
  • Seed Starch: Seeds from plants like Cycas rumphii and Dioon edule are processed into flour and cooked for consumption.
  • Edible Seeds: The seeds of Pinus gerardiana (chilgoza) are edible.
  • Kaffir Bread: A type of bread is prepared from the stem pith of Encephalartos.
  • Vegetable Use: Young leaves of Cycas are cooked and consumed as vegetables.
• Medicinal Value:
  • Ephedrine: Ephedrine, an alkaloid extracted from Ephedra, is used in the treatment of conditions like asthma, cough, cold, and bronchitis.
  • Cardiac Stimulant: Tincture of Ephedra serves as a cardiac stimulant.
  • Cycas Juice: The juice from young leaves of Cycas revoluta is employed to treat conditions like blood vomiting and flatulence.
• Industrial Use:
  • Gum: Cycas gum is used as an adhesive and as an antidote for snake bites. It is also applied to malignant ulcers.
  • Tannins: Tannins extracted from the bark of trees like Araucaria, Pinus, and Sequoia find use in the leather industry.
  • Canada Balsam: Canada balsam, derived from Abies balsamea, is used as a mounting medium in biological preparations.
  • Amber: Amber, a fossil resin from Pinus succinifera, has various applications. Pinus wood is used for doors, poles, beams, railway wagon flooring, etc.
  • Plywood: Podocarpus is used to make plywood, while the needles of Pinus and other conifers are used in manufacturing fiberboards for packing cases.
  • Paper Production: Wood pulp from Pinus, Picea, Abies, and Gnetum is used to create papers such as newsprints, writing papers, and printing papers.
  • Handicrafts: Cycad leaves are employed in crafting baskets, mats, hats, brooms, and other items.
  • Fiber Use: Fibers from Cycas and Macrozamia leaves are utilized for stuffing pillows and making mattresses.
  • Timber: Gymnosperms provide softwood that is used in crafting lightweight furniture, plywood, packing cases, matchsticks, railway sleepers, and more.
  • Linoleum: Sawdust from gymnosperms is used in the production of linoleum and plastics.
• Resin:
  • Resin is a semi-fluid substance that solidifies upon exposure to air, sealing injuries, cones, and buds. It's antiseptic and repels pests. Resin is commercially processed to obtain turpentine and rosin. Rosin is used for waterproofing, sealing joints, and making writing paper, while turpentine is employed as a solvent in paints, polishes, and wax. It also has medicinal applications for pain relief, bronchitis treatment, and expelling worms.
• Source of Oils:
  • Oils derived from the seeds of various gymnosperms like Cycas revoluta, Macrozamia reidlei, Pinus cembra, and Cephalotaxus drupacea serve as edible oils.
  • Red cedar wood oil from Juniperus virginiana is used for cleaning microscopic preparations and oil immersion lenses.
  • Oils extracted from Cedrus deodara, Cryptomeria japonica, and Cupressus sempervirens are employed in perfume preparation.