Plants with Seeds – Gymnosperms and Angiosperms

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Kingdom Plantae – Phanerogams – Plants with Seeds: Gymnosperms, Angiosperms: Monocots and Dicots or Dicotyledon and Monocotyledon.

Source | Credits | Picture Credits: NCERT General Science

Plantae - Plant Kingdom

Phanerogams – Plants with Seeds

  • Plants with well differentiated reproductive tissues that ultimately make seeds are called phanerogams.
  • Seeds are the result of the reproductive process. They consist of the embryo along with stored food, which serves for the initial growth of the embryo during germination.
  • This group is further classified, based on whether the seeds are naked or enclosed in fruits, giving us two groups: gymnosperms and angiosperms.

Gymnosperms

  • This term is made from two greek words: gymno– means naked and sperma– means seed.
  • The plants of this group bear naked seeds [ovules are not enclosed by any ovary wall] and are usually perennial, evergreen and woody. The seeds that develop post-fertilisation are naked too. Examples are pines, such as deodar.
  • Gymnosperms include medium-sized trees or tall trees and shrubs. One of the gymnosperms, the giant redwood tree Seguoia is one of the tallest tree species.
  • The roots are generally tap roots {Plant Parts and Their Functions}. Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cgcas) small specialised roots called coralloid roots are associated with N2-fixing cyanobacteria.
  • The leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity and wind.
  • In conifers, the needle-like leaves reduce the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.
  • The gymnosperms are heterosporous; they produce haploid microspores and megaspores.
  • The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones.

Kingdom Plantae - Gymnosperms - Naked Seeds (Mobile) (Custom)

  • The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili.
  • The microspores develop into a male gametophytic generation which is highly reduced and is confined to only a limited number of cells. This reduced gametophyte is called a pollen grain. The development of pollen grains take place within the microsporangia.
  • The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.
  • The male or female cones or strobili may be borne on the same tree (Pinus). However, in cycas male cones and megasporophylls are borne on different trees.
  • Unlike bryophytes and pteridophytes {Bryophytes – Pteridophytes }, in gymnosperms the male and the female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes.
  • The pollen grain is released from the microsporangium. They are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls.
  • The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharge their contents near the mouth of the archegonia.
  • Following fertilisation, zygote develops into an embryo and the ovules into seeds. These seeds are not covered.

Gymnosperms - Cycas - Pinus - Ginkgo (Mobile)

Figure: Gymnosperms: (a) Cycas (b) Pinus (c) Ginkgo

Angiosperms

  • This word is made from two greek words: angio– means covered and sperma– means seed.
  • Unlike the gymnosperms where the ovules are naked, in the angiosperms or flowering plants, the pollen grains and ovules are developed in specialised structures called flowers.
  • The seeds develop inside an organ which is modified to become a fruit. These are also called flowering plants.
  • The male sex organ in a flower is the stamen. Each stamen consists of a slender filament with an anther at the tip. The anthers, following Meiosis, produce pollen grains.
  • The female sex organ in a flower is the pistil or the carpel. Pistil consists of an ovary enclosing one to many ovules. Within ovules are present highly reduced female gametophytes termed embryo-sacs. The embryo-sac formation is preceded by meiosis. Hence, each of the cells of an embryo-sac is haploid.
  • Each embryo-sac has a three-celled egg apparatus – one egg cell and two synergids, three antipodal cells and two polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary nucleus.
  • Pollen grain, after dispersal from the anthers, are carried by wind or various other agencies to the stigma of a pistil. This is termed as pollination.
  • The pollen grains germinate on the stigma and the resulting pollen tubes grow through the tissues of stigma and style and reach the ovule.
  • The pollen tubes enter the embryo-sac where two male gametes are discharged. One of the male gametes fuses with the egg cell to form a zygote (syngamy).
  • The other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN).
  • Because of the involvement of two fusions, this event is termed as double fertilisation, an event unique to angiosperms.

Life cycle of an angiosperm (Custom)Life cycle of an angiosperm (Custom)

Figure: Life cycle of an angiosperm

  • The zygote develops into an embryo (with one or two cotyledons) and the PEN develops into endosperm which provides nourishment to the developing embryo.
  • The synergids and antipodals degenerate after fertilisation. During these events the ovules develop into seeds and the ovaries develop into fruit.
  • Plant embryos in seeds have structures called cotyledons. Cotyledons are called ‘seed leaves’ because in many instances they emerge and become green when the seed germinates. Thus, cotyledons represent a bit of pre-designed plant in the seed.

Monocots and Dicots

  • The angiosperms are divided into two groups on the basis of the number of cotyledons present in the seed.
  • Plants with seeds having a single cotyledon are called monocotyledonous or monocots. Plants with seeds having two cotyledons are called dicots.

Angiosperms - dicotyledon - monocotyledon (Mobile)

Figure: Angiosperms : (a) A dicotyledon (b) A monocotyledon

Kingdom Plantae – Summary

  • Plant kingdom includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
  • Algae [thallophytes] are chlorophyll-bearing simple, thalloid, autotrophic and largely aquatic organisms.
  • Depending on the type of pigment possesed and the type of stored food, algae are classfied into three classes, namely Chlorophyceae, Phaeophyceae and Rhodophyceae.
  • Algae usually reproduce vegetatively by fragmentation, asexually by formation of different types of spores and sexually by formation of gametes which may show isogamy, anisogamy or oogamy.
  • Bryophytes are plants which can live in soil but are dependent on water for sexual reproduction. Their plant body is more differentiated than that of algae. It is thallus-like and prostrate or erect and attached to the substratum by rhizoids. They possess root-like, leaf-like and stem-like structures.
  • The bryophytes are divided into liverworts and mosses. The plant body of liverworts is thalloid and dorsiventral whereas mosses have upright, slender axes bearing spirally arranged leaves.
  • The main plant body of a bryophyte is gamete-producing and is called a gametophyte. It bears the male sex organs called antheridia and female sex organs called archegonia.
  • The male and female gametes produced fuse to form zygote which produces a multicellular body called a sporophyte. It produces haploid spores. The spores germinate to form gametophytes.
  • In pteridophytes the main plant is a sporophyte which is differentiated into true root, stem and leaves. These organs possess well-differentiated vascular tissues. The sporophytes bear sporangia which produce spores.
  • The spores germinate to form gametophytes which require cool, damp places to grow. The gametophytes bear male and female sex organs called antheridia and archegonia, respectively. Water is required for transfer of male gametes to archegonium where zygote is formed after fertilisation. The zygote produces a sporophyte.
  • The gymnosperms are the plants in which ovules are not enclosed by any ovary wall. After fertilisation the seeds remain exposed and therefore these plants are called naked-seeded plants.
  • The gymnosperms produce microspores and megaspores which are produced in microsporangia and megasporangia borne on the sporophylls. The sporophylls – microsporophylls and megasporophylls – are arranged spirally on axis to form male and female cones, respectively. The pollen grain germinates and pollen tube releases the male gamete into the ovule, where it fuses with the egg cell in archegonia. Following fertilisation, the zygote develops into embryo and the ovules into seeds.
  • In angiosperms, the male sex organs (stamen) and female sex organs (pistil) are borne in a flower. Each stamen consists of a filament and an anther. The anther produces pollen grains (male gametophyte) after meiosis. The pistil consists of an ovary enclosing one to many ovules. Within the ovule is the female gametophyte or embryo sac which contains the egg cell.
  • The pollen tube enters the embryo-sac where two male gametes are discharged. One male gamete fuses with egg cell (syngamy) and other fuses with diploid secondary nucleus (triple fusion). This phenomenon of two fusions is called double fertilisation and is unique to angiosperms. The angiosperms are divided into two classes – the dicotyledons and the monocotyledons.
  • During the life cycle of any sexually reproducing plant, there is alternation of generations between gamete producing haploid gametophyte and spore producing diploid sporophyte. However, different plant groups as well as individuals may show different patterns of life cycles – haplontic, diplontic or intermediate.
Match the following (column I with column II)
Column I Column II
Chlamydomonas Moss
Cycas Pteridophyte
Selaginella Algae
Sphagnum Gymnosperm
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