The gymnosperms are a group of seed-producing plants which are also known as Acrogymnospermae. The term ‘Gymnosperm’ is originated from two Greek words, ‘gymnos’ meaning naked and ‘sperma’ meaning seed. It is a smaller ancient group of plants that produce naked seeds because their seeds are not enclosed by a fruit. On earth, more than 1000 gymnospermic plants species are still found. The notable groups are cycads (Cycas, Lepidozamia, Macrozamia, Zamia, Microcycas, etc), conifers (cypresses, pines, cedars, firs, junipers, larches, redwoods, kauris, etc), gnetophytes (Ephedra, Gnetum, and Welwitschia) and Ginkgo (Ginkgo biloba). Among them, the largest group of living gymnosperms is Conifers while ginkgo is a single living plant species which is found in China. Generally, gymnosperms are plentiful in the temperate forest zone and they can tolerate dry or moist conditions. They have needle-like leaves and most of them are evergreen. In the Himalayas of the Indian subcontinent, they are more abundant and form coniferous forests. Sequoiadendron giganteum (giant saquoia: family-Sequoioideae) is popularly known as a redwood which is the largest conifer species that can grow more than 100 meters in height.
The gymnosperms belong to six phyla:
- Cycadophyta
- Ginkgophyta
- Gnetophyta
- Pinophyta (Coniferophyta)
- Pteridospermales and
- Cordaitales
Among the above six phyla, Pteridospermales and Cordaitales have gone to extinct.
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Salient Characteristic Features of Gymnosperms
- All gymnospermic plants are terrestrial and most of them are tall, woody, perennial, evergreen trees or shrubs and very few are climbers.
- The gymnosperms are always green all year round and continue alive for a long period.
- Gymnosperms bear stem and root system but do not bear ovary and stigma. In this case, roots absorb water and different types of minerals from the soil while the stems help to transport the minerals.
- Gymnosperms are unisexual and are woody type trees which bear needle-like leaves to reduce water loss.
- They possess xerophytic characteristics because they have thick bark, thick cuticle, thick hypodermis, sunken stomata, scales leaves, transfusion tissue, etc. Some have needle-like leaves or small leathery leaves.
- The main plant body is sporophyte (2n) and autophyte. In this case, the sporophyte plant body is differentiated into root, stem, and leaves.
- The stem is erect, woody, branched (Pinus) or unbranched (Cycas). In most cases, the stem is excurrent (Pinus) and leaf scar is present over the stem.
- Leaves may be of one type or two types: dimorphic like green foliage leaves and dry brown scale leaves. The foliage leaves are evergreen, simple or compound and are covered with a thick cuticle.
- The seeds of gymnosperms are naked and cone-bearing in nature which develops either on the cones or scale (leaf-like appendages).
- The root system consists of a branched trap root. Sometimes, algae and fungi inhabit within the roots as endophytes (Pinus). Roots may be associated with stronger associated roots. When a root contains a mycorrhizal fungus, then it is called the mycorrhizal root. The root hairs are scanty.
- Gymnosperms are flowerless plants which create seeds and cones. Seeds of all gymnosperms are enclosed in cones for protecting their seeds. In this case, male cones create pollen (male gametophyte) and female cone create eggs. Hence, the male is called pollen cone while the female is called a seed cone.
- Xylem and phloem make the conducting tissue of root, leaves, and stem. Collateral endarch open vascular bundles are found in the stem. In the case of the root system, the vascular bundles are radial where the vascular cylinder is diarch to polyarch. Secondary growth also occurs in the stem and root system.
- Xylem is composed of bordered pitted tracheid and xylem parenchyma. In most of the cases, xylem vessel or trachea absent but it is present in Gnetum. Phloem is composed of sieve-tube and phloem parenchyma but companion cells are absent.
- In many cases, transfusion tissue is present in the internal structure of a leaf. Sunken stomata are also present in the leaves.
- Flowers of gymnosperms are simple. In nature, they are unisexual and naked because they do not enclose by fruits. Perianth or calyx and corolla are absent in most of the gymnosperms but they are present in Gnetum.
- They are heterosporous because they produce two types of spores such as haploid microspores and megaspores. Microspores are produced from the micro-sporangia while megaspores are produced from the megasporangia. These sporangia are formed special leaf-like structures which are known as microsporophylls and megasporophylls.
- The male flower of many gymnosperms is composed of the only microsporophyll while the female flower is composed of the only megasporophyll.
- Numerous microsporophylls are compactly arranged together to form a compact cone-like structure which is known as male strobilus or male cone while the megasporophylls are compactly arranged together to form a compact cone-like structure which is known as female strobilus or female cone. But in Cycas, sporophylls do not form any compact female cone. Each strobilus is composed of similar type of sporophylls.
- Two or more microsporangia (sing. Microsporangium) are found on the lower surface of each microsporophyll.
- They produce two types of gametophytes. Microspore on germination produces male gametophyte and megaspore on germination produces female gametophyte. Gametophytes are formed within the spore case (micro and megasporangium).
- All gymnosperms are anemophilous and pollen grains (microspores) are directly carried by wind to the micropyle of the ovule during pollination. In this case, the pollen grains fall directly on the pollen drop and then they are sucked into the nucleus.
- Male gametes may be either ciliated and motile in Cycas or non-ciliated and non-motile in Pinus.
- Endosperm formation takes place within the female gametophyte before fertilization where the endosperm tissue is haploid.
- The female sex organs are known as archegonia which are formed in the female gametophyte. Female gametophytes supply nutrition to the developing embryo.
- The lower part of the zygote develops into an embryo. Sometimes more than one embryo is formed in each ovule. In this case, this phenomenon is known as polyembryony.
- After fertilization, the zygote develops into the embryo. In this case, embryo on germination produces root, stem, and leaf. The number of cotyledons varies from one to many.
- Ovules become true naked seeds, no formation of fruits occur.
Cycas revoluta
Pinus wallichiana
Ginkgo biloba
Gnetum macrostachyum
Concluding Remarks
Gymnosperms are an archaic group of the naked seeded plant. During the Mesozoic period (225 million years ago), they were most abundant. At present, they form only a small part of green vegetation. They have major economic values due to their various uses. Some gymnospermic conifer plants such as Pine, spruce, fir, and cedar are used for the production of lumber, paper, and resin. Some common items such as food, soap, varnish, gum, nail polish, and perfumes are also produced from many gymnospermic plants.
Some genera of gymnosperms such as Pinus bear fungal association roots (mycorrhiza) which play an important role in plant nutrition, soil chemistry, and soil biology while some other species (Cycas) have coralloid roots which contain cyanobacteria for helping nitrogen fixation.
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