Introduction
The seed plants (spermatophyta) are divided into two sub groups based on whether or not an ovary wall surrounds their ovules. The two sub groups of seed plants are Gymnosperms and Angiosperms.
The word Gymnosperms used by the Theophrastus, pupil of Aristotle for the first time in his famous book “Enquiry into Plants” and is derived from the Greek word, (‘gymnos’ –naked, ‘sperma’-sperms) meaning naked seed. Gymnosperms are plants in which ovules are freely exposed, both before and after fertilization. In other words, an ovary wall does not surround the ovules of gymnosperms and even the seeds that developed post-fertilisation are naked.
Examples
General characteristics of Gymnosperms
- Plants are sporophytes, having true roots, stems and leaves.
- They bear naked seed that are not enclosed in an ovary.
- They have sporangia born on sporophyll which together forms Stobili or cones.
- Strobili or cones responsible for spores production.
- All forms are heterosporous and the two kinds of spores produced male and female gametophytes.
- Gametophytes are small and dependent on sporophytes for nutrition.
- Gymnosperms contain vascular tissue, but they have no vessels in their xylem and have no companion cell and sieve tubes in their phloem.
- Single fertilisation occur i.e. one sperm involve in fertilisation.
Classification of Gymnosperms
Morphology
The gymnosperms comprise of only a small portion of the plant kingdom with approximately 70 genera and 725 species include trees (tall and medium sizes) and shrubs. The tallest trees in the plant kingdom belong to the gymnosperms. One of the gymnosperms, the giant redwood tree Sequoia is one of the tallest trees. The smallest gymnosperm is a Zamia pygmaea (Cycad).
Morphology of roots
The roots are generally tap roots in gymnosperms but sometimes they have symbiotic association with fungus in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with nitrogen(N2) fixing cyanobacteria.
Morphology of stems
The stem are aerial, branched (Pinus, Cedrus) or unbranched (Cycas, Zamia). In gymnosperms, the branching may be of two types (i) long shoot with unlimited growth (ii) dwarf shoot with limited growth, as seen in Ginkgo.
Morphology of leaves
The leaves show great variation in the form and arrangement. Gymnosperms leaves may be large (megaphylls) or small (microphylls).The leaves may be simple or compound. The shape of leaves also ranges from pinnate or fern-like in Cycas or needle-like in Pinus. The leaves of Ginkgo are fan-shaped. The leaves in Gymnosperms are well- adapted to withstand extremes of conditions such as temperature, humidity and wind. Conifers such as Pinus have needle-like leaves which reduce the surface area. Their thick cuticle and sunken stomata (most gymnosperms show xerophytic characters) also help to reduce water loss.
The arrangement of leaves on the stem is highly variable may be whorled (Cedrus), opposite and decussate (Gnetum, Ephedra) or spiral (Taxus, Podocarpus ).
Gymnosperms show diversity in leaf venation, it may be single veined in most genera to reticulate (Gnetum), parallel (Welwitschia), or even dichotomous (Ginkgo).
Reproduction of Gymnosperms
The life cycle of gymnosperms exhibit alternation of generation, that is, they spend part of their lives in the multicellular diploid sporophyte stage and part in the multicellular haploid gametophyte stage. The sporophyte generation is the dominant stage in this group.
Vegetative propagation is rare in this group but reproduction by bulbils is known in Cycas.
Sexual Reproduction
Gymnosperms are heterosporous producing two kinds of spores- haploid microspores and megaspores in different spore-bearing structures (microsporangium and megasporangium or ovule).
The sporangia borne on sporophylls, leaf-like scales which are arranged spirally along an axis to form compact Strobili or Cones. The strobili bearing microsporophyll and microsporangia are called microsporangiate or male strobili, while the strobili bear megasprophylls with megasporganium(ovule) are called macrosporangiate or female Strobili.
Male Strobili / Microsporangiate – Microsporangia are borne on the lower surface of the microsporophyll. Microsporangia further contain numerous microsporocytes also called microspore mother cells. Each microsporocyte undergoes meiosis to form haploid microspores. Microspores developed into highly reduced male gametophyte called pollen grain. The development of pollen grain take place within the microsporangium.
Female Strobili / Macrosporangiate – Megasporangium borne on the upper surface of the megasporophyll. The megasporocyte or megaspore mother cells are differentiated from one of the cells of the nucellus. The nucellus is a parenchymatous mass of cells that protected by envelop called integuments and the composite structure called ovule.
Megasporocytes undergo meiosis to produce four haploid megaspores. Only one divides mitotically, to form female gametophyte that bears two or more archegonia or female sex organ that contain female gamete (egg).Other three megaspores are nonfunctional and degenerate soon. The multicellular female gametophyte is also retained within the megasporangium. In Gymnosperms, gametophytes are endosporic i.e. they develop within a spore wall.
Pollination and fertilisation
Pollen grain is carried to the ovules by wind and this is followed by fertilisation. The transfer of pollen grains to the female cones is called pollination. Once the ovule is ready to receive pollen grains, it produces sticky droplet(Pollination drop) at the opening which capture wind-borne pollen.
One of the many pollen grains that adhere to sticky female cone, develop pollen tube. The pollen tube carrying the male gametes grows toward the archegonia and discharges their contents near the mouth of the archegonia. The male gamete fuses with the egg to form zygote. Following fertilisation, zygote develops into embryo and the ovules into the seeds. These seeds are naked.
