All autotrophic, chlorophyll-producing, multicellular organisms are classified under the “Plant Kingdom”.
A brief introduction to the Plant Kingdom
Plants are present all around us but there is more to them than meets the eye. This kingdom is made up of a diverse number of organisms, each with its unique characteristics and properties. Plants are usually lauded for their ability to produce oxygen which is what sustains life on our planet. Without plants, there would be no chance of our survival. It is important to keep this in mind when thinking about environmental conservation.
A typical plant cell
Even though there is a wide variety of plants types and sub-types, all plants share certain common features, which will be elucidated as follows-
- The plant cell wall which provides structural integrity and support to the cell is typically made up of cellulose, hemicellulose and pectin. Some other complementary components include suberin and lignin which provide additional support and protection to plant cells. The presence of a cell wall is what makes a plant cell visibly distinct from an animal cell.
- The plant cell membrane lies under the cell wall and is made up of a thin layer of fats and proteins. This cell membrane restricts the entry of toxic substances into the cell while also maintaining the osmotic conditions inside the cell and enabling intracellular nutrient transport.
- The nucleus in a plant cell performs the same functions as in most eukaryotic cells. It is a membrane-bound organelle that is the storehouse of nucleic acids such as DNA and RNA. It is also responsible for the production of proteins and ribosomes. The membrane of the nucleus is perforated to allow the movement of proteins and these pores are called ‘nucleopores’.
- Plastids are membrane-bound organelles that possess their own circular DNA. They are of three types, namely-leucoplasts, chloroplasts and chromoplasts. Leucoplasts store fats and oils. Chromoplasts store plant pigments and chloroplasts contain chlorophyll which is one of the main requirements for the process of photosynthesis.
- The central vacuole in a plant cell is considerably bigger than the vacuoles seen in animal cells. It contains cell sap and is made up of various salts and enzymes. Its main function is the storage of cell sap. It is also responsible for the maintenance of cell turgor pressure.
- Plant cells possess all the cell organelles present in most eukaryotic cells including the golgi apparatus, mitochondria and lysosomes. It is important to note that plant cells do not possess centrosomes.
Visualisation of plant cells under a microscope.
Divisions of the Plant Kingdom
The Plant Kingdom is classified into different divisions based on the presence or absence of the following features-
- Fully-differentiated plant body.
- Ability to bear and produce seeds.
- Ability to produce fruits.
- Transport-specific tissues.
This division is the lowest in the hierarchy of the Plant Kingdom. Plants that belong to this kingdom are commonly referred to as algae. The body of these plants is not well-differentiated into a stem, root system and leaves. Such plants are found only in aquatic or moist habitats. Some examples include Chara, Spirogyra and Chlamydomonas.
Microscopic visualisation of Chlamydomonas cells.
Commonly known as the “amphibians of the Plant Kingdom”, these plants are terrestrial with a certain degree of structural differentiation. They possess stem-like, leaf-like and root-like structures. They are found in swampy areas and can live both on land and in water. Some examples include Marchantia and Funaria.
A species of Bryophytes.
Evolutionarily, these plants were the first to exhibit the development of a vascular system for the transport of water and nutrients. These plants have well-differentiated root and shoot systems. They do not produce seeds, but instead produce spores, also called ‘naked embryos’ due to the absence of a seed coat. Ferns are the most commonly found pteridophytes.
Fiddlehead or Ostrich ferns
These plants are characterised by their seed-bearing ability and well-developed vascular system. They also have well-developed reproductive systems and are classified as ‘higher plants’. These plants are well-differentiated into the root, shoot and leaves. They are further classified into two subtypes, namely, gymnosperms and angiosperms. This classification is based on the presence or absence of a seed coat.
- Gymnosperms- these plants have a well-developed vascular system but lack vessels. They do not produce covered seeds or form fruits. The formation of a strobilus or a ‘cone’ is the most evident characteristic of this sub-division. These plants are perennial, meaning they remain green throughout the year. They commonly have woody stems. There are around 650 species of gymnosperms.
Strobili of a gymnosperm plant.
- Angiosperms- also known as flowering plants, possess the ability to bear fruits and flowers. They produce seeds that are covered. These plants may be annual, perennial or biannual meaning they bloom once a year, throughout the year or twice a year respectively. They have a well-developed vascular system with xylem and phloem components. Based on the number of cotyledons present, angiosperms can be categorised as monocotyledons (one seed leaf) and dicotyledons (two seed leaves).
Orchids as an example of monocot plants.
Bellflowers as an example of dicot plants.
The Plant Kingdom is incredibly diverse with a multitude of plant types, each with its unique features. Plants not only produce oxygen but also provide food, medicines and other raw materials required for us to survive and thrive. Plants support the growth and survival of every other organism on the plant and hence, should be conserved and protected. Plants are a marvel of nature and exhibit a high degree of complexity in their physiological, morphological and metabolic functioning.