Classification of Plant Kingdom
Posted on : 24-11-2017 Posted by : Admin

To study the plants in a systematic and scientific manner, classification is very important. Based on the first phylogenetic system of classification proposed by Eichler, the plant kingdom is divided into two sub-kingdoms namely Cryptogams and Phanerogams.

 

SUB KINGDOM: Cryptogams

These are flowerless, seedless spore bearing plants. In other words, these plants never bear flowers, fruits and seeds. They reproduce asexually by spores and sexually by gametes. This sub kingdom is divided into the following three divisions

 

DIVISION: Thallophyta

It is the largest division of cryptogams. These are the simplest and most primitive form of plants. All of them have thallus body which is not differentiated into root, stem and leaves.

  • The plant body may be unicellular or multicellular.
  • Vascular tissues are absent.
  • Sex organs are usually unicellular and non-jacketed.
  • Embryo is not formed as the zygote divides by meiosis to form meiospores.
  • Life cycle is haplontic.

 

DIVISION: Bryophyta

They are chlorophyllous, autotrophic, embryophytic, atracheophytic, archegoniate and amphibious cryptogams which show invariable heteromorphic alternation of generation in their haplodiplontic life cycle.

  • They are Primitive land plants that grow on moist shady places. Though they started land life, they require the presence of water to complete their life cycle. They need water for the movement of motile male gametes.
  • They exhibit heteromorphic alternation of generations in which the haploid gametophyte is dominant phase and the diploid sporophyte is physically and nutritionally dependent on the gametophyte.
  • The Gametophyte is other thalloid or leafy form having stem-like central axis and leaf like appendages and rhizoids
  • The plant body consists of simple parenchymatous tissue.
  • The gametophyte reproduces sexually by zooidogamous type of oogamy
  • The sex organs are multicellular with sterile jacket. Female gametangium is known as archegonium and male gametangium as antheridium which have multicellular stalk.
  • The fertilized egg cell formed in venter of archegonium divides mitotically to form embryo that develops into sporophyte.
  • The sporophyte produces meiospores of only one kind
  • Spore is the first cell of gametophytic generation
  • The spore germinates to produce gametophytic plant either directly or through a juvenile filamentous stage called protonema
  • Life cycle is haplodiplontic with heteromorphic alternation of multicellular generations.

 

DIVISION: Pteridophyta

They are chlorophyllous, autotrophic, embryophytic, tracheophytic, archegoniate cryptogams which show diplohaplontic life cycle.

  • These are true land plants in the evolution of plant kingdom
  • Diploid sporophyte is the dominant phase in the life cycle.
  • The sporophyte is differentiated into true roots, stem and leaves. The leaves may be small microphyllous or large macrophyllous
  • All the vegetative parts of sporophyte possess vascular tissues, organized into stele or vascular bundles. So, Pteridophytes are the first tracheophytes in the evolution of plant kingdom. They are the only vascular cryptogams
  • Sporophyte produces meiospores inside the sporangia; the sporangia are borne on the lower surface of fertile leaves called sporophylls. It may be of two types, homosporous (only one type of spores) heterosporous (two types of spores)
  • Development of the sporangium may be leptosporangiate (Sporangium develops from a single superficial cell) Eusporangiate (Sporangium originates from a group of superficial cells.)
  • The spore germinates to produce a haploid gametophyte called prothallus. Homosporous species usually produces bisexual gametophytes whereas heterosporous species produce unisexual gametophytes.
  • Microspore germinates to produce male gametophyte and megaspore female gametophyte.
  • The gametophyte performs sexual reproduction by zooidogamous type of oogamy
  • Sex organs are multicellular with a sterile jacket but without stalks
  • Fertilization takes place in the venter of archegonium. And water is very essential for fertilization
  • The diploid zygote develops into embryo in the archegonial venter. The embryo grows into sporophyte.
  • The life cycle is diplohaplontic that shows heteromorphic alternation of generation of sporophyte and gametophyte which are independent of each other.

 

SUB KINGDOM: Phanerogams

These are seed producing tracheophytes. For the purpose of reproduction they bear flowers or cones. They show heteromorphic alternation of generation in which sporophyte is dominant phase and gametophyte is reduced. The gametophyte is totally dependent on sporophyte for nutrition. In Phanerogams, multicellular seed is formed with an embryo. These plants are terrestrial with well-organized sporophyte that has complex vascular tissues. For the sexual reproduction in these plants water is not essential. This sub kingdom has only one division, Spermatophyta.

 

DIVISION: Spermatophyta

Spermatophytes are the plants which bear seeds but they may or may not bear fruits. The spermatophytes are further divided into two sub divisions,

 

SUBDIVISION: Gymnospermae

These are Phanerogams without ovary and fruit. The seeds of these plants are naked without fruit wall. The following are the salient features of these plants:

  • Gymnosperms are perennial, evergreen and woody plants
  • The plant is sporophyte which has well-developed tap root system, woody stem and macrophyllous leaves.
  • These plants have vascular tissue. Vessels are absent in xylem and companion cells are absent in the phloem.
  • These plants are heterosporous. Sporophylls are usually aggregated to form compact structures called cones or strobili
  • The microsporangium produces microspores. The pollination is direct and anemophilous
  • Ovules are orthoporous and remain exposed on the megasporophyll. Female gametophyte formed from haploid megaspore is called endosperm. It produces aechegonia and also gives nutrition to embryo. So endosperm is haploid and is pre-fertilization product.
  • Sexual reproduction is siphonoamous type of oogamy. Male gamets are carried to the archegonia by pollen tube.
  • The diploid zygote develops into an embryo while still enclosed within the ovule.

 

SUBDIVISION: Angiospermae

These are Phanerogams with ovary and fruits. The seeds of these plants are enclosed in the fruit wall. These are fruit bearing flowering plants. The following are the salient features of these plants:

  • Angiosperms are herbaceous or woody plants that grow in almost every kind of habitat.
  • These plants produce flowers with usually conspicuous perianth
  • Microsporophylls are called stamens. Each stamen consists of filament and an anther with two or four microsporangia that produce pollen grains.
  • Megasporophyll is called carpel and is differentiated into ovary, style and stigma. Ovules are produced inside ovary.
  • Pollination is indirect. It may be zoophilous, anemophilous or hydrophilous.
  • The female gametophyte produced from haploid megaspore is inside the ovule and is called embryo sac. It contains a single egg cell
  • Siphonogamous type of oogamy takes place in the embryo sac.
  • Double fertilization takes place in these plants. Syngamy results in the formation of diploid zygote and triple fusion results in a triploid primary endosperm nucleus which divides to form a special nutritive tissue known as endosperm. Hence, the endosperm in angiosperms is triploid and is a post-fertilization product.
  • After fertilization, ovaries ripen into fruit and ovules into seeds which are enclosed by fruit wall. The embryo inside the seeds may have one or two cotyledons.

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