Main divisions of Plant kingdom are:
1. Algae :
(i) Chlorophyta (Green Algae)
(ii) Phaeophyta (Brown Algae)
(iii) Rhodophyta (Red Algae)
2. Bryophyta (Liverworts and Mosses)
3. Tracheophyta (Vascular plants)
A brief description of these is given below—
Algae : Algae have three main divisions, namely—Rhodophyta (red algae), Phaeophyta (brown algae), Chlorophyta (green algae).
Algae is divided into three different kingdoms. They all lack any specialised tissue and have a thallus like body. Plant kingdom includes red, brown and green algae.
The divison Rhodophyta has red algae. These algae contain a red pigment phycoerythrin and a blue pigment anthocyanin along with chlorophyll and generally brownish or greenish blue in colour. Most of these algae are marine and usually found in warmer tropical seas. They have a cellulosic cell wall containing phycocolloids. Some contain calcium carbonate and have coral-like structures. Some exhibit alternation of haploid and diploid generation. They do not form flagellated cells at any stage.
The filamentous forms have holdfast to attach to the substratum. Some algae are of economic importance.
Phaeophyta includes brown algae. Brown colour is due to the presence of pigment fucoxanthin present along with chlorophyll. Most of these algae are found in cooler seas and some which grow up to 30 metres are called as kelps e.g., Laminaria. They have a photosynthetic lamina, stem like stipe and holdpast to anchor to the rocks below.
The phycocolloids present in the cell wall does not allow drying or freezing.
Some show alternation of generation with the diploid phase being dominant. In sexual reproduction flagellate male gamete get formed.
Chlorophyta includes green algae. It contains chlorophyll a and b. Mainly found in fresh water could be unicellular or multicellular. Stores starch like higher plants. The land plants are supposed to have come out from green algae.
Chlamydomonas is a unicellular flagellated motile algae having a cup shaped chloroplast and an eye spot. The sexual reproduction is by zygote meiosis. In this flagellated gametes are formed. The asexual reproduction is by the formation of zoospores.
A filamentous green algae having a band shaped chloroplast is called ulothrix. It reproduces by asexual reproduction by flagellated zoospores.
Spirogyra is a filamentous algae with spiral shaped chloroplast in each cell. Sexual reproduction is by conjugation and formation of zygospore.
Liverworts and mosses belong to Bryophyta. These are mainly found in moist and shady places. They do not have true roots, stems and leaves. The gametophyte is dominant.
Moss plant Funaria is a gametophyte which develops from a haploid spore. The gametophyte bears archegonia and antheridia which produce female and male gametes respectively. The fertilization leads to formation of zygote which gives rise to the sporophytic generation. It is dependent on the gametophyte. It exhibits alternation of generation.
Tracheophyta contains all vascular plants. The plant body has specialized conducting tissues xylem and phloem. Their dominance on earth is due to their deep roots, leaves covered with cutin and woody tissues.
Ferns are primitive vascular plants. The plant body is a sporophyte with the gametophyte reduced to a simple thallus. The leaves of ferns have various shapes. They are called as fronds. The sporophyte is diploid and grows for many years in contrast to gametophyte which is haploid, short lived and inconspicuous.
Gymnosperms are seed plants without flowers. Fertilisation needs rain water or dew. Cycads and Conifers like Pinus are the common examples.
The development of secondary growth, the evolution of pollen grains and the seed led to their success.
A fertilized ovule having three generations is called a seed.
In Pinus the female cone takes three years for maturity. The pollination occurs in the 1st year, fertilization in the 2nd year and dispersal in the 3rd year.
Angiosperms are known as flowering plants. The enclosed seeds, presence of flowers and fruits, adaptations to different habitats and the development of conducting tissue has led them to dominate the earth for the last 70 million years.
The size may range from 1 m.m. Wolffia to 100 m Eucalyptus.
They may be annuals, biennials or perennials. The herbs, shrubs or trees come in this category.
Angiosperms are of two types, namely monocots and dicots on the basis of number of cotyledons present. Their structure of root, stem, leaf and flower are different.
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