GYMNOSPERMS
- Gymnosperms are seed plants in which the seeds remain exposed over the surface of the megasporophylls.
- The plant body is differentiated into root, stem and leaves.
- The plants possess well developed tap root system. In some cases, the roots are symbiotically associated with algae (e.g., coralloid
roots of Cycas) or with fungus (e.g., mycorrhizal roots of Pinus). - The stem is erect, aerial, solid, woody and branched.
The gymnosperms include world’s tallest tree-Sequoia sempervirens measuring about 125m in height and 30m in girth.
The smallest gymnosperm is Zamia pygmaea, which has underground tuberous stem. It reaches only upto 25cm in height. - The vascular tissues, xylem and phloem are present. They are arranged in vascular bundles. The vascular bundles of stem are
conjoint, collateral and open. Xylem vessels are absent. - Secondary growth occurs and annual rings are distinct in most of the gymnosperms.
- Gymnosperms are heterosporous.
- The two types of sporangia are borne on special leaf-like structures called sporophylls. The microsporangia are borne on
microsporophylls (= stamens) and the megasporangia on megasporophylls (= carpels). - The sporophylls are aggregated in the form of cones or strobili.
- The microsporangium produces numerous light pollen grains.
- Pollination is anemophilous. Some of the pollen grains come in direct contact with the micropylar end of ovules and reach upto
the nucellus. - Fertilisation occurs by siphonogamy, i.e., the male gametes reaches the female gamete by forming a tube.
- The diploid zygote, develops into an embryo while still enclosed within the ovule and the ovule becomes a seed.
- During seed development, the endosperm (haploid) supplies nutrients to the developing embryo.
- The seeds of gymnosperm are naked and remain attached to the megasporophyll.
- Polyembryony is common in Pinus. The number of cotyledons may be one or two or a whorl of many.
- Gymnosperms can be classified into following types:
Gymnosperms
Cycadopsida- Primitive group, represented by small plants.
- Wood is manoxylic.
- Male cones are large with compactly arranged microsporophylls.
- Megasporophylls are loosely arranged in female cone.
- Seeds are radially symmetrical.
- Examples : Cycas, Zamia, etc.
Gnetopsida- Includes advanced gymnosperms.
- Secondary xylem shows vessels.
- Ovules are orthotropous with
long tubular micropyle. - Examples : Order Gnetales
(Gnetum, Ephedra, Welwitschia)
Coniferopsida- Includes larger dominant
gymnosperms. - Wood is pycnoxylic.
- Sporophylls form cones.
- Seeds are bilaterally symmetrical.
- Examples : Pinus, Ginkgo, Taxus.
Economic Importance of Gymnosperms
- Gymnosperms are a very good source of resins. The resins are oxidation products of various essential oils containing terpenes,
resin acids and esters. The resins are utilised commercially in the manufacture of varnishes, waterproof paints, sealing wax etc. - Seeds of Pinus gerardiana are used as dry fruits, called chilgoza.
- Ephedrine, a drug is obtained from Ephedra. It is used in the treatment of cough, asthma and bronchitis.
- Leaves of Cycas and Thuja are extensively used in the floral decorations.
Adaptations to land
Seed plants, i.e., gymnosperms and angiosperms are the most successful of all the land plants. They are well adapted to land
conditions because of the following features.- Presence of extensive underground root system for anchoring and absorption of water and minerals.
- Presence of vascular tissue system for conduction of water and minerals.
- Well developed mechanical tissue system.
- Development of cambium for secondary growth in thickness.
- Production of pollen grains in large number.
- Transformation of megasporangium into ovule and development of female gametophyte inside the ovule.
- Presence of external water is not required for fertilisation.
- Zygote develops within the ovule to form an embryo.