In this article we will discuss about:- 1. Characteristics of Gymnosperms 2. Types of Living Gymnosperms 3. Economic Importance.
Characteristics of Gymnosperms:
Gymnosperms are, those seed plants in which the seeds remain exposed over the surface of the megasporophylls because the latter are not folded to form pistils.
1. Gymnosperms are a small group of seed plants which are represented by only 900 living species.
2. Gymnosperms are more ancient than the angiosperms. They formed dominant vegetation on earth some 200 million years back in mesozoic era. Today they are dominant only in cold areas, where instead of rain; snow is the source of water.
At other places they have been replaced by angiosperms. In warmer areas only a handful of gymnosperms can be observed, e.g., Cycas (like C. circinalis in South India), Araucaria (native of South America, New Zealand and East Australia, like A. heterophylla.
3. All gymnosperms are perennial and woody, forming either bushes or trees. Some of these are very large and live for thousands of years, e.g., Sequoia sempervirens (tallest gymnosperm of 111.6 m) Zamia pygmia is smallest (26 cm).
4. Flowers are absent. Two types of sporophylls, microsporophyll’s and megasporophylls are usually aggregated to form distinct cones or strobili, pollen cones (male cones) and seed cones (female cones) respectively.
5. Seeds do not occur inside a fruit. They are naked.
6. A distinction of ovary, style and stigma is absent.
7. Ovules are orthotropous and sessile. Each ovule is surrounded by a 3-layered integument.
8. Female gametophyte contains archegonia.
9. Pollination is direct as a stigma is absent and the pollen grains directly reach the micropylar ends of ovules. Pollination is usually accomplished by wind (anemophily).
10. Male gametophyte produces only two male gametes or sperms. Generally one of them is functional.
11. An external water is not required for transport of male gametes. Instead, a pollen tube is formed by the male gametophyte for effecting fertilization (siphonogamy).
12. Seeds contain a food laden tissue or endosperm for future growth of embryo into seedling. The tissue represents the female gametophyte.
13. Like pteridophytes, xylem does not possess vessels except in some geophytes. Phloem is without companion cells and sieve tubes. Sieve cells are not arranged end to end in rows.
14. Vascular tissues are arranged into vascular bundles just like angiosperms. Vascular bundles of stem are open so that secondary growth is quite common.
Types of Living Gymnosperms:
Living gymnosperms are divided into four groups— Cycadopsida, Ginkgopsida, Coniferopsida and Gnetopsida. Ginkgopsida is represented by maiden hair tree (Ginkgo biloba).
Gnetopsida contains Gnetum, Ephedra and Welwitschia. These are nearer to flowering plants in having flower like arrangement of sporophylls and possessing primitive vessels in xylem. Plants of cycadopsida and coniferopsida are commonly called cycads and conifers respectively.
It is smaller group of gymnosperms which have palm-like habit and fern-like foliage. Leaf bases are often persistent. In stem the vascular tissues constitute only a narrow patch though secondary growth occurs. Plants are dioecious, that is, microsporophyll’s and megasporophylls develop on separate plants. The sporophylls are aggregated to produce strobili or cones (exception megasporophylls of Cycas).
Megasporophylls are often leaf-like and possess ramenta. Sperms are motile. The plants are mostly xerophytic and occur in warm weather. Their population is, however, decreasing due to extension of agriculture and severe competition from xerophytic angiosperms. Cycads are grown for their ornamental appeal, e.g., Cycas circinalis, C. revoluta.
Cycas (Fig. 3.18):
Cycas is also called living fossil because it possesses a number of characters of extinct pteridosperms and cycads. The gymnosperm is an evergreen small palm-like or tree-fern like sporophyte that occurs in some tropical and subtropical areas.
It reaches a height of 0.4 m to 20.0 m. Cycas has an un-branched columnar stem (rarely branched) which is covered by spiral bands of persistent rhomboidal leaf bases. The top bears a crown of leaves.
Leaves are of two types, large green foliage and small brownish scale leaves. Scale leaves develop in spiral rows alternating with foliage leaves. They are covered with ramental hair. Foliage leaves are large (1-3 m), petiolate and unipinnate. Base is broad. Petiole bears two rows of spines. Petiole continues into rachis. Rachis has two lateral grooves from which arise 50- 200 pairs of sessile, linear-lanceolate pinnae or leaflets.
Leaflet apex is spiny while the margin can be straight or revolute (e.g., C. revoluta, C. beddornei). Young leaves show circinate ptyxis. Roots are of two types, normal and coralloid.
Coralloid roots are irregular, negatively geotropic, dichotomously branched coral like roots which do not possess root hairs and root caps. Coralloid roots have a symbiotic association with blue-green algae like Nostoc and Anabaena species. Sago grains occur in the stem cortex.
Vegetative reproduction occurs by fleshy bulbils which arise adventitiously in the crevices amongst persistent leaf bases. Sexually Cycas is dioecious with distinct male and female plants. The plants bear terminal cones. Male cone is oval, thick, 20-80 cm long stalked structure which replaces the growing point of the plant. Further growth of male plant is carried out by a lateral bud, so that stem axis of male plant is sympodial.
Male cone has a number of densely crowded spirally arranged microsporophyll’s. Each microsporophyll has a flat fertile proximal region and a distal bent sterile region or apophysis. The fertile region bears a number of microsporangia or pollen sacs arranged in sori. Microspores or pollen grains are boat-shaped.
Female cone is loose. It consists of a rosette of brown megasporophylls. Each megasporophyll bears 2-12 reddish ovules in the middle fertile part. The terminal sterile part may show remains of foliar structure. Ovules of Cycas are the largest, 6-7 cm in diameter. Similarly, egg or oosphere is the largest in plant kingdom.
Pollination is anemophilous. Before pollination, pollen grain develops three cells prothallial cell, generative cell and tube cell. Pollination brings pollen grains inside the ovules. The tube cell grows and forms a pollen tube. Pollen tube is haustorial in function in Cycas. Generative cell divides into sterile stalk cell and large fertile body cell. Body cell produces two sperm mother cells.
The sperms of Cycas (upto 300 pm) are the largest in the biological world. They are top-shaped with a number of flagella present in 5-6 grooves on the pointed end. Seed is oval to rounded orange-red structure which generally has a fleshy outer sarcotesta for attracting birds.
They comprise more than 500 living species of cons bearing gymnosperms which have withstood competition from angiosperms. The plants are evergreen with dense and massive vascular tissues and non-motile gametes. Conifers are usually monoecious. Cones or strobili are compact and woody. Both the types of cones are borne on the same plant. Unlike cycads, they are not produced on the tips of main branches.
Conifers have monopodial growth where the main axis of trunk rises straight from base and reaches up to the tip. The branches do not compete with the main axis. Stem branches are of two types, long and dwarf. Growth of the long branches is indefinite. The lower long branches are longer than the upper ones. This gives a conical or ex-current appearance to the plant just like a ‘Christmas Tree’.
Leaves are mostly borne on the dwarf branches. The dwarf branches have a limited or definite growth. Leaves have thick cuticle and sunken stomata. They are needle like (e.g. Pinus), small flat and leathery (e.g., Araucaria) or scale like (e.g. Thuja). All these leaf characters help conifers in conserving water and reduce transpiration in dry conditions associated with cold habitat.
The roots are often associated with fungal symbionts or mycorrhizae so as to overcome difficulty in absorbing water and obtaining inorganic nutrients from organic debris.
The plants usually possess an aromatic antiseptic sticky semifluid substance called resin. It is secreted by special channels or ducts present in wood, bark, leaves, roots, etc. Resin seals the places of injury and protects the plants from attack of microbes as well as insects.
Araucaria (a tall and majestic conifer of South America, New Zealand and East Australia) grows in warm weather in plains as well as foot hills.
Other conifers form vast and dominant woodlands in mostly north temperate regions of Europe, Asia, North America, etc. They are abundant on the upper timber-line zone of mountains including the Himalayas, e.g., Pinus (Pine), Cedrus (Cedar), Picea (Spruce), Abies (Fir), Juniperus (Juniper), Thuja, Larix, Cupressus.
Conifers are dominant constituents of north temperate flora due to the following reasons:
1. In the temperate areas, conifers have an advantage over angiospermic trees. While the angiospermic trees shed their leaves during autumn-winter period, the conifers remain evergreen. They continue to manufacture their food during this period when other plants are in a state of hibernation because of the absence of leaves.
2. Conifers have a number of xerophytic characteristics which help in conserving water. They are, thus, able to tide over the winter period when the soil becomes frozen and water availability is very little. Mycorrhizae allow them to get water and minerals.
3. Enzymes of conifers are functional even at —35°C at which temperature they become inactivated in other plants.
Pinus (Fig. 3.18):
Pinus or Pine is a coniferous gymnosperm. It is a large tree having pyramidal or ex-current shape like a Christmas tree. The plant body is a sporophyte having stem, leaves and roots.
The main stem is straight. It may reach a height of 10-50 metres. The main stem shows monopodial branching. Stem branches are of two types, long and dwarf. The dwarf branches possess 1-5 needle-like foliage leaves which are surrounded at the base by a sheath of scale leaves.
Pinus has a horizontally spreading tap root system with peg-like downwardly directed roots present at intervals. Finer roots are of two types— normal (with root hairs and root cap) and mycorrhizal. The mycorrhizal roots occur near the soil surface. They are devoid of root hairs and root cap.
The sporophytic plant body does not multiply vegetatively. It produces micro- and megasporophylls in two types of cones, male and female. Male or pollen cones occur in clusters sub-terminally on lower long branches. Each male cone has a short stalk, a central axis and a number of spirally arranged microsporophyll’s.
A microsporophyll bears two oblong, parallel microsporangia on its lower surface. Pollen grains are dispersed by air currents. They form yellow clouds in the pine forests. A pollen grain has two air sacs or wings for making it light.
The female or seed cones develop in groups of 2-6 on upper long branches of the tree. Each female cone has a long stalk and a central axis on which are borne a number of spirally arranged paired scales. The lower of the pair is called bract scale while the upper scale is ovuliferous scale or megasporophyll. The ovuliferous scale bears two ovules towards the basal region on the upper side.
The female cones open in the year of their formation for pollination. Pollination is anemophillous (by air) and direct. After fertilization the ovule matures into a seed.
Part of the upper surface of the ovuliferous scale is peeled off along with the seed to form its wing. A female cone takes about 26 months for reaching maturity. It then opens to release winged seeds which are dispersed by air. After falling on a suitable soil, each seed gives rise to a new plant.
They are primitive gymnosperms which possess deciduous leaves with furcate venation, two types of branches, unisexual plant, catkin like male inflorescence with each microsporangiosphore having 2-12 microsporangia, multi ciliate male gametes and clusters of megasporangiate structures each with a long stalk and two ovules. Ginkgoales are represented by a single species, Ginkgo biloba.
Ginkgo biloba (Fig. 3.19) has not changed for the last several millions of years since its appearance in triassic period. It is also called living fossil. The plant has survived due to interest shown by horticulturists. It has natural immunity to several plant diseases. The plant is a tall tree of upto 30m height. Leaves are fan-shaped.
They are deeply bilobed on long shoots. The leaves are entire or sinuate on dwarf shoots. Leaves possess furcate venation. Ginkgo is dioecious like cycads. Dwarf shoots of female plants bear distinct megasporangiate structures.
Each has a stalk that ends into two ovules. Ovule has a 3-layered integument, micro Pyle and a pollen chamber. Endosperm of roasted seed is edible. However, horticulturists prefer to grow only male plants as the female plants give unpleasant smell.
Economic Importance of Gymnosperms:
Some of the Major Economic Importance’s of Gymnosperms are given below:
1. Edible Seeds:
Seeds of Pinus gerardiana are edible. They are eaten after roasting.
Gymnosperms possess softwood. The same is used in preparation of light furniture, plywood, packing cases, match sticks, railway sleepers, etc.
A number of gymnosperm woods are used in the manufacture of paper. They include Picea, Pinus, Larix and Abies.
4. Fibre Boards:
Needles of Pinus and other conifers are used in making fibre boards that are used in making packing cases.
Saw dust is employed in making linoleum and plastics.
Resin is a semifluid secreted by special tubes which contains terpenes, resin acids and esters. It solidifies on exposure to air. Therefore, it plugs the places of injury. It helps in sealing female cones after pollination, scale leaves around leaf bases and apical buds. Resin retains water. It is antiseptic and toxic to pests.
Therefore, it prevents microbial and insect attack. Resin is commercially extracted and distilled to obtain turpentine and rosin. Rosin is used in water proofing, sealing joints and preparation of writing paper. Turpentine is used as solvent in paints, polishes and wax. It is employed medicinally in removing pains, curing bronchitis and expelling worms.
Drug ephedrine is obtained from Ephedra, a gymnosperm. The drug is used in curing respiratory ailments, including asthma.