Gymnosperms: The gymnosperms are spermatophytes i.e. seed-producing plants. The Gymnosperms (Gymno: naked, Sperma-seed) are the form of a plant in which the ovules are not enclosed by the ovary wall.
- In these plants the ovule remains uncovered i.e. naked before and after fertilization. After fertilization, developed seeds are also naked.
- Gymnosperms are the smallest group of the plant kingdom.
- Smallest Gymnosperm- Zamia
- Tallest Gymnosperm– Sequoia giganteum
Plant body in gymnosperms:
Medium-sized trees or tall trees and shrubs come under Gymnosperms. The giant redwood Sequoia is one of the tallest gymnosperm trees. The main plant body is sporophytic, differentiated into roots, stems, and leaves.
Roots: Generally tap roots are found in Gymnosperms. In some genera, the roots having fungal association forms mycorrhiza (Pinus), in some others (Cycas) coralloid roots are associated with N2- fixing cyanobacteria. In roots Radicle vascular bundles present with xylem.
Stem: Maybe unbranched (Cycas) or branched (Pinus, Cedrus). in stem conjoint, collateral, and open vascular bundles with endarch xylem is present.
Leaves: May be simple or compound dimorphic, foliage, and deciduous scaly leaves. The leaves may be microphyllous or megaphyllous. Well-adapted for resisting temperature, humidity, and wind.
- In Cycas, the pinnate leaves persist for a few years.
- In conifers, the needle-like leaves reduce the surface area, and the thick cuticle and sunken stomata help to reduce water loss.
Life Cycle of Gymnosperms
The gymnosperms are heterosporous, and they produce microspores and megaspores, produced within sporangia, born on sporophylls that are spirally arranged to form compact strobili or cones.
The microspores get developed into a male gametophytic generation which is confined to a limited number of cells, that gametophyte is called a pollen grain.
- Within the microsporangia the development of pollen grains takes place.
- Macrosporangia or female strobili are the cones bearing megasporophylls.
- In trees like Pinus, the male and female cones may be borne on the same tree, and in Cycas male and female cones are borne on different trees.
- The envelopes Protect the nucellus and the composite structure called the ovule. Ovules are borne on megasporophylls and clusters to form a female cone.
- The mother cell (megaspore) divides meiotically and forms four megaspores. Megasporangium develops as multicellular female gametophyte bearing archegonia (female sex organ).
- Male and Female gametophytes are dependent on each other.
- Microsporangium releases pollen grains, carried by air, and comes in contact with megasporophylls. The pollen tube grows towards archegonia in the ovule and discharges that male gamete near the funnel of archegonia.
- The zygote develops after fertilization into an embryo and the ovule develops into seeds that remain naked.
Alternation of Generation in gymnosperm
In gymnosperms, a different alternation of generation is found. Gametophytic generation is reduced and the development is upon Sporophytic generation.
Classification of Gymnosperms
The gymnosperms are divided into two subclasses:
The Cycadophytae are small plants consisting of unbranched stems, monoxylic wood, large and compound foliage leaves. Sporophylls are grouped in cones. Male gametes in gymnosperm usually motile. It is divided into three orders:
- Cycadofilicales or pteridospermales or pteridosperms (seed sperms)- connecting link between Pteridophytes and Gymnosperms. e.g. Medullosa and Lyginopteris.
- Fossil cycads: The leaf base of these plants are permanently attached to the stem, e.g. Williamsonia, and Williamsoniella.
- Cycadales or Living Cycads: having Palm like appearance. e.g. Cycas (sago palm) Microcycas, etc
The Coniferophytae are large trees with a branched stem and pycnoxylic wood, and simple foliage leaves. These are tall woody plants. Coniferophytes are perennials. These are evergreen plants. It is divided into four orders
- Cordaitales: in form of fossils e.g. Cordaites, and poroxylon.
- Ginkgoales: Ginkgo biloba (maiden hair tree)
- Coniferales: Largest order of modern gymnosperms, e.g. Pinus (pine), Podocarpus, Sequoia (redwood tree), Araucaria (Christmas tree or monkey’s puzzle), Cupressus, etc.
- Gentales: highest order of gymnosperms, connecting link between gymnosperm and angiosperm, as a flower-like arrangement of sporophyll is found. This is the only order of gymnosperm in which vessels are present in the xylem with tracheids. e.g. Gentium, Ephedra
Economic Importance of Gymnosperms
- Chilgoza the seed of the plant Pinus giardiana is very important as it is used as dry fruit.
- The softwood of gymnosperms is used in preparation light furniture, plywood, matchstick, railways sleepers, etc. Eg. of plant Cedrus deodara
- From the Ephedra, the drug ephedrine is obtained.
- The very energy provider, starchy food sago is obtained from the stem of Cycas revoluta, also called sago palm.
- A mounting agent ‘Canada balsam’ which is used in permanent slide preparation, is obtained from Abies balsamae.
- From Juniperus Virginiana, cedar oil is obtained. Cedar oil is used in microscopy.
- An anticancerous chemical Taxol is obtained from Taxus.
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