PLANT KINGDOM CLASSIFICATION | 5 Major Divisions in Plant Biology

In this tutorial, we will learn about the Plant Kingdom Classification which is one of the major topics in Plant Biology.

All living organisms are classified into five broad kingdoms – Monera, Protista, Fungi, Animalia and Plantae.

PLANT KINGDOM CLASSIFICATION

Previously many members of Fungi, Monera and Protista which had cell walls were classified under Plantae. At that time any living organism with a cell wall was classified under Plantae. But after careful experiments, it was found though they all had cell walks the composition of their cell walls was widely different.

So they were placed under separate kingdoms. A popular exam is the blue-green algae which have now been moved to the Monera kingdom and are called cyanobacteria.

PLANT KINGDOM CLASSIFICATION

Division -Thallophytes

This includes algae. The organisms don’t have a differentiated body(thalloid). They are autotrophic and contain chlorophyll. The algae reproduce by vegetative(fragmentation), asexual(spores)and sexual(gametes) methods.

  • Thallophytes are divided into the 3 classes- Chlorophyceae – green algae, store food as starch,  cellulosic walls, chlorophyll a and b  are the major pigments, 2 to 8 flagella present
  • Phaeophyceae- brown algae, store food as mannitol and laminarin, cellulosic and pectin walls, chlorophyll a, c and fucoxanthin are the major pigments, 2 flagella present
  • Rhodophyceae –red algae, store food as floridean starch, walls made of cellulose, pectin and polysulphate esters, chlorophyll a, d and phycoerythrin are the pigments, no flagella

Bryophytes

They need both water and soil to complete their life cycle and are knowns as amphibians of the plant kingdom. They may have root-like(rhizoids), leaf-like or stem-like structures. Haploid plant body produces gametes (gametophyte). The male sex organ is called the antheridium. While the female is archegonium. When the antherozoids are released into water, they fuse with the archegonium. The zygote produces a sporophyte attached to the gametophyte (dependent). The sporophyte is divided into the foot, seta and capsule.

This produces haploid spores that germinate into a gametophyte. Bryophytes are divided into 2 classes-

  • Liverworts- Dorsiventral thalloid gametophyte, the gametophyte is close to the substrate, both asexual ( fragmentation or gemma cups ) and sexual reproduction may occur
  • Mosses- More complex sporophyte, the stages are protonema (creeping, green, branched ) and leafy stage(bears sex organs). Asexual reproduction by fragmentation or budding. The leafy stage is upright with spirally arranged leaves.

Pteridophytes

These are the first terrestrial plants to possess vascular tissues namely the xylem and phloem. These vascular tissues are well differentiated so that each cell can perform its required function. The spread of pteridophytes is limited as they require water for the transfer of the antherozoids (male gametes).  Pteridophytes have true roots, stem and leaf structures. The dominant phase of pteridophytes is sporophyte. If small leaves are present they are called microphylls while larger leaves are called macrophylls . Sporangia surrounded by leaf-like sporophylls are formed on the sporophyte. These sporophylls form a compact structure called cone or strobilus. The sporangia produce spores by meiosis. These spores germinate into small, multicellular, independent thalloid gametophytes called

prothallus. The gametophyte produces the male (antheridium)  and female (archegonium) sex organs. The female gametophytes of these plants are retained over the parent sporophytes and zygote develops into the embryo within this gametophyte.

The antherozoids released by the antheridium are transferred by water to the mouth of archegonium where it fuses with the egg. This fusion forms the zygote which divides to produce multicellular sporophyte. Spores formed may be of equal sizes (homosporous) as in most cases. Or also as one large (mega)  and one small (micro) which are heterosporous. These megaspores and microspores germinate to give rise to female and male gametophytes, respectively.

Gymnosperms

In gymnosperms, the ovules are naked and are not enclosed within any ovary wall at any point in time. Gymnos stands for naked and sperms mean seeds.

Gymnosperms usually have taproots. In some cases, these roots have fungal(mycorrhiza) association or have a symbiosis with nitrogen-fixing cyanobacteria(coralloid roots). Gymnosperms grow under extreme conditions (temperature, rain, humidity) and thus they have modified leaves that are suitably adapted. Conifers have needle-like leaves that reduce the surface area to prevent water loss. The leaves also have a thick cuticle and sunken stomata to combat the dry conditions.

All gymnosperms are heterosporous and produce haploid microspores and megaspores within sporangia that are surrounded by sporophylls. These sporophylls are spirally arranged along an axis to make the strobili or cones. The male or female cones can be borne on the same tree or different trees.

The male strobili contain the microsporophyll and are known as the microsporangium. These male spores develop into the male gametophyte which is contained within the Sporangia. Thus in gymnosperms, the gametophyte body is highly reduced (dependent,  very few cells). The male gametophyte is called a pollen grain. The female strobili contain megasporophylls and is known as megasporangium. The megaspore mother cell is formed from the nucellus. The nucellus along with the megaspore mother cell is known as the ovule. The megaspore mother form four haploid megaspores as the cell undergoes meiotic cell division. Three megaspores degenerate and one develops into a multicellular female gametophyte. The multicellular female gametophyte is retained within megasporangium and is dependent. The pollen grain released from the microsporangium enters the opening of the ovules on megasporophylls. These pollen tubes carrying the male gametes grow towards archegonia in the ovules and contents are discharged at the mouth of the archegonia. After fertilization, the zygote develops into an embryo and the ovules develop into seeds.

Angiosperms

The ovules are present within seeds and seeds are not naked. The male sex organ stamen. consisting of a slender filament with an anther bearing pollen grains. The female sex organ is the pistil consisting of an ovary at the base, a style, and stigma. The ovary contains ovules each having an embryo sac. Each embryo-sac has one egg cell and two synergids, three antipodal cells and two polar nuclei. The anther release pollen grains which travel by pollination and land on stigma. Pollen grains germinate on stigma and enter into the embryo sac. Each pollen has two male gametes. One fuses with the egg to form a zygote and other fuses with the 2 polar nuclei to form triploid primary endosperm nucleus (PEN). This is called double fertilization. The PEN develops into an endosperm. This endosperm provides nourishment to the developing embryo. The other cells degenerate. The ovules form the seeds and the ovaries develop into a fruit. Angiosperms are classified into –

  • Monocots -seeds having a single cotyledon, parallel venation in leaves, trimerous flowers having 3 (or multiple of 3) members in each whorl
  • Dicots- two cotyledons in each seed, reticulate venation in leaves, pentamerous flowers having 5 (or multiple of 5) members in each whorl.
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