R.H. Whittaker organized the organisms into five kingdoms. He classified organisms on the basis of cell structure, mode, source of nutrition and body design. The five kingdoms proposed by Whittaker are Monera, Protista, Fungi, Plantae, and Animalia. Let’s learn about the plant kingdom i.e., kingdom Plantae.
Plant Kingdom – Plantae
Plantae is the plant kingdom which includes all plants on the earth. They are multicellular eukaryotes. Typically, they consist of a rigid structure that surrounds the cell membrane called the cell wall. Plants also have a green coloured pigment called chlorophyll that are quite important for photosynthesis. Hence, they have an autotrophic mode of nutrition. The plant kingdom is a vast group; therefore, the kingdom is further classified into subgroups. Level of classification is based on the following three criteria:
- Plant body: whether the body has well-differentiated structures or not.
- Vascular system: whether the plant has vascular system for transportation of substances or not
- Seed formation: whether the plant bears flowers and seeds or not; if it does, then whether it is enclosed within fruits or not.
Considering all these factors, the plant kingdom has been divided into five subgroups. They are as follows:
All the plants that lack a well-differentiated body structure belong to the subgroup Thallophyta. Thallophytes are commonly known as algae. The majority of them are aquatic. Some examples are Spirogyra, Chara, Ulothrix, etc.
Bryophytes have differentiated plant body like stem, leaf structures. But they lack a vascular system for the transportation of substances across the plant body. Bryophytes are found in both land and aquatic habitats, hence are known as amphibians of the plant kingdom. Mosses and Marchantia belong to this subgroup.
Pteridophytes have well-differentiated structures such as stem, root, leaves as well as a vascular system. Ferns, horsetails, Marsilea are some common examples of Pteridophytes.
Gymnosperms are plants that have well-differentiated plant body, vascular system and they bear seeds. The term is derived from Greek words, gymno: naked and sperma: seed. The seeds of gymnosperms are naked which means they are not enclosed within a fruit. The perennial, evergreen woody trees belong to this group. Pines, deodar, redwood, etc. are few examples.
Angiosperms are also seed-bearing plants with well-differentiated plant body. The word is derived from Greek words: angio: covered and sperma: seed. Unlike gymnosperms, seeds of angiosperms are enclosed inside the fruits. Angiosperms are commonly known as flowering plants. Examples include the Mango tree, pomegranate plant, etc. Seeds germinate from embryonic leaves called cotyledons.
Depending on the number of cotyledons present in seeds, angiosperms are divided into two: monocotyledons (or) monocots and dicotyledons (or) dicots.
Cryptogams and Phanerogams
The plant kingdom has been classified into two groups ‘cryptogams’ and ‘phanerogams’ based on their seed formation ability.
Cryptogams are plants that do not have well-developed or conspicuous reproductive organs. They have hidden reproductive organs and don’t produce seeds. The thallophytes, the bryophytes and the pteridophytes are ‘cryptogams’. Reproduction in all the three groups occurs through spore formation.
Plants that have conspicuous reproductive organs and produce seeds are called ‘phanerogams’. Gymnosperms and Angiosperms belong to the group phanerogams.
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