Two main divisions of the plant kingdom had been recognized since long which are:
Classification of plant kingdom
1) Cryptogams (Kryptos-concealed; gamos-marriage) :
These plants never bear flowers and seeds (flowerless-seedless plants). Reproduction in these plants takes place by spores. This division include three main groups:
1. Thallophyta : They are simplest and most primitive plants. The plant body is an undifferentiated mass of cells known as Thallus. It is not differentiated into roots, stem and leaves. Sexorgans may be absent or are usually unicellular and non-jacketed. Embryo is not formed, Vascular tissue is absent.
This group includes, Algae, Fungi, Bacteria and Lichens (A peculiar combination of an alga and fungus living symbiotically).
2. Bryophyta : In this group the plant body is more advanced than thallophyta, some bryophytes have thalloid plant body (Liverworts and Hornworts) whereas in Mosses the plant body is differentiated into stem-like and leaf-like structures and rhizoids. Roots are absent. The sex organs are multicelled with a sterile jacket of cells. They are the most primitive land plants and are called ‘amphibians of plant word’. Vascular tissue is absent. An embryo is always present.
3. Pteridophyta : These plants have well differentiated plant bodies, with roots (some exceptions), stem and leaves (some exceptions). They possess vascular tissue and therefore, called vascular cryptogams, sexorgans are multicellular with sterile sheath. Distinct embryo is always present.
2) Phanerogams or Spermatophyta (Phaneros-visible; gamos-marriage) :
These plants bear flowers and seeds (so called-Spermatophyta). Two main groups are included in Phanerogams.
1. Gymnosperms (Gymos-naked, sperma-seed) : Gymnosperms are naked seeded plants i.e., their ovules (=seeds) are not enclosed by ovary walls (=fruit wall), i.e., their ovules are borne on the surface of megasporophylls. They have very simple flowers, without accessory whorls and microsporophylls and megasporophylls are aggregated in cones (no female cone in Cycas). The ovules are directly pollinated by the pollen grains (microspore). They do not have ovary, style and stigma. They do not have fruits.
2. Angiosperms (Angeion-a case) : In the angiosperms the ovules(=seeds) are enclosed in the ovary wall(-fruit wall — pericarp). These plants bear flowers with conspicuous accessory whorls. Megasporophyll is also called carpel and is differentiated into ovary, style and stigma. The pollen grains (microspores) are lodged on stigma (pollination) and the ovary developing into fruit.
Angiosperms are placed into two subdivisions—Monocotyledons and Dicotyledons depending upon the number of cotyledons in the embryo.