Unquestionably, the diversification of organisms on the face of the earth is the most beautiful display of the power of nature. The first step humanity took to understand this beauty was with the tool of science. The systematic classification of the organisms has led us to comprehend the vividness of their differences and similarities. This classification has been the foundation for the growth of other biological sciences. The great North American ecologist Robert Harding Whittaker made the most crucial step in this direction. He divided the organisms into five primary kingdoms – Monera, Protista, Fungi, Plantae and Animalia.
Plants are the strongest and deepest link between humans and earth. Here and there, we observe plants have benefited humans in all parts of life. From the basic needs of food, oxygen, shelter, and clothing to the advanced energy requirements via fossil fuel, plants help us everywhere. Hence, it becomes imperative to undertake a detailed study into the Kingdom Plantae. Let us glance at some of the characteristic features of plants below.
You can also read this blog to know more about plants.
What makes the Kingdom Plantae so special?
- The foremost point is that these are non-locomotive beings.
- They possess a green pigment called chlorophyll that helps them create their food. Hence, plants are autotrophs.
- Their mode of reproduction is asexually by vegetative propagation or sexually.
- Plants are multicellular eukaryotes. They possess a rigid outer cell wall and a large central vacuole.
- Also, they have distinct organelles for different functions like anchoring, reproducing, support and photosynthesis.
How to further classify Plant Kingdom Plantae?
The plant kingdom is pretty vast. This has led to the need for a further subdivision of plants. For this very purpose, here is the list of factors used in their segregation.
- Plant Body: The physical structure is a great differentiating point. Revolving around on the presence or lack of body parts like root, stem and leaves.
- Vascular System: The fluid transportation system of a plant is a highly significant factor. Plants are divided because they have a well-defined system like phloem and xylem.
- Seed Formation: Further, plants also differ in their method of procreation. Whether they have flowers and seeds and are the seeds naked or protected inside a fruit are strong factors.
What are the main categories of plants?
Considering the factors mentioned above and features, the plants are subdivided into five subgroups as follows;
- Thallophyta
- Bryophyta
- Pteridophyta
- Gymnosperms
- Angiosperms
Thallophyta
Thallophyta is polyphyletic, i.e. they have more than one common evolutionary ancestor. They have a very simple and primitive body build thallus and can be filamentous, colonial, branched or unbranched. Their common examples include algae, fungus, lichens, bacteria etc.
Defining Features of Thallophyta
- Thallophytes lack true roots and vascular tissue needed to transport water and minerals. Hence they are mostly found in moist and wet places.
- The majority of their members can generate their food and are autotrophic. However, certain members like fungi are dependent on other sources of food.
- After photosynthesis, the produced glucose is immediately consumed. Any leftover part is converted and stored in the complex compounds called starch.
- A cell wall made up of cellulose surrounds its cells.
- They do not have vascular tissues like phloem and xylem.
- As the sex organs are simple and single-celled, there is no embryo formation after fertilisation.
Thallophyta is further separated into Algae and Fungi.
Algae are simple, chlorophyll bearing autotrophic, thalloid and largely aquatic organisms. Their reproduction method is vegetatively by fragmentation and sexually by gametes formation that can show isogamy, anisogamy, or oogamy. Algae are organized into Chlorophyceae, Phaeophyceae, and Rhodophyceae. They are likewise seen to develop symbiotic relations with sloths native to rainforests of South America and Central America. The moisture-retaining fur affords a nourishing environment for the algae to thrive, and in return, the Algae presents the sloth with nutrients and camouflage from predators.
Fungi
Fungus is a heterotrophic thallophyta, which is also achlorophyllous, i.e. unable to produce chlorophyll. For obtaining their food, fungi often take the help of symbiosis with other algae and cyanobacteria. In return, the fungi provide them with shelter against the sun’s harmful UV rays. A great example of this is Lichen, where the fungi and algae act together as a single unit.
Bryophyta
Their lack of vascular tissues primarily characterises bryophytes. Since they can originate in land and aquatic environments, they have been termed amphibians of the plant kingdom. Bryophytes are believed to have developed from charophytes and thus considered as the first true plants ever developed. Their common examples include Mosses, Marchantia, Liverworts and Hornworts.
Defining Features of Bryophytes
- They have crude stems and leaves. Although lacking true roots, they have rhizoids for anchoring to the surface.
- Unlike the true roots, rhizoids are incapable of absorbing nutrients.
- Mosses tend to release spores from their leaves that travel via water and create new mosses in new places.
- Water is a necessity for them to grow and spread. In the case of the arid region, they can survive by suspended animation and then revive again when brought in contact with water.
Bryophytes are the same as other embryophytes with the alternation of generations. Haploid gametophyte cells have a fixed number of unpaired chromosomes. This gives rise to a diploid sporophyte that contains twice the number of paired chromosomes. Diploid zygotes are formed by combining haploid sperm and eggs produced by gametophytes. Diploid zygotes grow into a sporophyte.
Pteridophyta
Initially, before plants had developed flowers, fern-like plants were found everywhere for hundreds of millions of years. Pteridophytes are quite similar to these types of plants. The striking distinction between Pteridophytes and other plants is that they reproduce through spores instead of seeds.
Division of Pteridophyta
Pteridophyta is classified into four main classes:
Psilopsida
- They are the most primitive.
- Stems are photosynthetic and dichotomously branched.
- Rhizoids are present.
- Leaves are mostly absent.
- The sporophyte is a homosporous synangium.
- Examples include Psilotum and Tmesipteris.
Lycopsida
- They are commonly known as club moss.
- They possess a differentiated plant body with adventitious roots, stem, rhizophores and leaves.
- The sporophyte is homosporous or heterosporous.
- Examples include Selaginella and Lycopodium.
Sphenopsida
- They are mostly known as horsetails.
- They possess a distinctive plant body with roots arising from nodes of the underground rhizome, stem and scaly leaves.
- Homosporous, sporangia are borne on strobili.
- Examples include Equisetum.
Pteropsida
- They are very commonly known as a fern.
- They own a well-defined plant body with roots, stem and leaves.
- The sporophyte is homosporous or heterosporous.
- Antherozoids are multiflagellate.
- Examples include Pteris, Dryopteris, and Adiantum
Defining Features of Pteridophyta
- Most people speculate that life originated in the oceans. Pteridophytes are believed to be the first plants to have evolved and adapted to live in the lab.
- Pteridophytes do not have seeds. They reproduce via spores. Although they possess vascular tissues, they lack xylem vessels and phloem companion cells.
- They have a very well defined structure of their body that consists of root, stem and leaves.
- Pteridophytes have sporangium, which is the structure where spores are created. They are homosporous and heterosporous, i.e. capable of making one or two kinds of spores.
- Sporangia are created in groups of sporophylls, which are nothing but the leaves that bear the sporangia. The leaf’s tips curl inwards to protect the vulnerable growing parts.
- They possess multicellular sex organs. Male organs are called antheridia, whereas female ones are termed as archegonia.
- True alternation of generations is seen here. Sporophyte and gametophyte generations are visible in Pteridophytes. The diploid sporophyte is the main plant body.
Pteridophytes show alternation of generations with a similar life cycle as seed-bearing plants. Differing from mosses and seed plants, both the haploid gametophyte and diploid sporophyte generations are independent and free-living. The sexuality of pteridophytic gametophytes is further classified as follows:
- Dioicous: the individual gametophyte is either male-producing antheridia and sperm or female-producing archegonia and egg cells.
- Monoecious: every individual gametophyte may produce both antheridia and archegonia, and it can function both as a male and a female.
- Protandrous: the antheridia mature before the archegonia.
- Protogynous: the archegonia mature before the antheridia.
Gymnosperms
Gymnosperms have a distinguishing plant body complete with vascular structure and bearing seeds. Their name is derived from the Greek words gymno, meaning naked and spermatic as seeds, which are not protected within a fruit. Their examples include evergreen woody trees like pines, deodar, redwood etc.
Defining Features of Gymnosperms
- Gymnosperms have undergone vast evolution and display some selective features such as lack of flowers and naked open seeds. Since they do not produce flowers, hence fruits are also absent here.
- The wind is the primary source of their self-pollination and dispersal of seeds. Usually, they are medium to tall, with a few shrub classes also available. Sequoia, a kind of gymnosperm, is one of the biggest and tallest tree species.
- They possess well defined and distinctive body parts. The leaves are needle-like in structure with thick cuticles and sunken stomata commonly seen in conifers for saving up on water loss during transpiration.
- The root system is primarily taproot type, which in certain plants are associated with fungi to form mycorrhiza, for example, in Pinus. In the case of extra species like Cycas plants, they have specialised roots called coralloid related to nitrogen-fixing cyanobacteria. Additionally, they are vascular with the presence of both xylem and phloem.
Also Read: Difference between Angiosperms and Gymnosperms
Angiosperms
Angiosperms have received their name from the Greek words angio, meaning confined, and sperma, meaning seed. Similar to gymnosperms, they have seeds and a well-distinguished plant body; however, the seeds, in this case, are enclosed inside fruits. These are flowering plants, and the seeds produced from embryonic leaves are called cotyledons. Examples consist of the mango tree and pomegranate plant.
Every angiosperm plant includes a vascular bundle equipped with xylem and phloem to transport water, minerals and nutrients. A well-defined body structure with a root system, shoot and leaves. Remarkably characteristic features are the flowers where male and female gametes mature where, after pollination, these flowers turn into fruits containing the seeds.
Angiosperms have a wide variety in their habitats and sizes. They encompass the tiny Wolfie to the large Australian mountain ash tree of 100 metres in height. Several plants are tall woody trees, shrubs and likewise herbaceous plants and develop further adaptations to their roots, stems and leaves, depending on their habitat requirements.
Division of Angiosperms
Based on the types of cotyledon existing, angiosperms are classified into two classes – monocotyledons and dicotyledons. The dicotyledonous angiosperms have two cotyledons in their seeds, and the monocotyledonous angiosperms contain one cotyledon.
Monocotyledonous Plants
The monocots contain adventitious roots, simple leaves with similar venation, and trimerous flowers. Vascular bundles are present in a larger quantity and closed. Examples include bamboos, bananas, sugarcane plants, cereals, lilies, etc.
Dicotyledonous Plants
The dicotyledonous plants have two cotyledons with a taproot arrangement with a reticulate venation in leaves. Flowers are either tetramerous or pentamerous. Vascular bundles are arranged in a ring between two and six in number. Examples include Grapes, sunflowers, Tomatoes, Dandelions, and Potatoes.
Secondary Division of Plants – Cryptogams and Phanerogams
Plants are also categorised as cryptogams and phanerogams upon their seed development capability.
- Cryptogams are those plants that do not possess a well developed or visible reproductive system and procreate via spores as they don’t create seeds. Thallophytes, Bryophytes, and the Pteridophytes fall into this category.
- Phanerogams are plants with a well-developed and visible reproductive system and are capable of producing seeds. Gymnosperms and Angiosperms have their place in the group phanerogams.
FAQs on Plant Kingdom
- Who is responsible for the five-kingdom classification of organisms?
The North American ecologist Robert Harding Whittaker was the first to divide the organisms into five separate categories.
- What are the five divisions of the plant kingdom?
The plant kingdom is divided into Thallophyta, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms.
- What are the deciding factors for the subdivision of the plant kingdom?
The deciding factors for the subdivision of the plant kingdom are the plant body, vascular system and seed formation.
- Which is the largest kingdom in plants?
Angiosperms are the largest division in the plant kingdom.
- What pigment is responsible for the creation of food in plants?
Chlorophyll, a green pigment found in the leaves of the plants, allows them to create food via photosynthesis.
Also, read the blog on Structure of plant cell and structure of Animal Cell to prepare for NEET 2022.
