Kingdom Plantae includes all the plants on the earth. They are multicellular, eukaryotes and consist of a rigid structure that surrounds the cell membrane called the cell wall. Plants also have a green colored pigment called chlorophyll that is quite important for photosynthesis.
The plant kingdom has the following characteristic features:
A plant kingdom is a vast group; therefore, the kingdom is further classified into subgroups. Levels of classification are based on the following three criteria:
Plant body: whether the body has well-differentiated structures or not.
Vascular system: whether the plant has a vascular system for the 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 classified into five subgroups. They are as follows:
All the plants that lack a well-differentiated body structure belong to the subgroup Thallophyta.
Thallophytes: Primitive plants where the body is not distinguished into stem, roots and leaves
They normally include members with primitive and simple body designs such as green algae and brown algae. The majority of them are aquatic. Common examples are Spirogyra, Chara, Ulothrix, etc.
Bryophytes: Small, non-vascular plants that choose moist environments
Bryophytes have distinguished 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: Spore-dispersing vascular plants
They 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: Vascular plants that possess “exposed” seeds
They 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 a few examples.
Angiosperms: Vascular plants that possess special characteristics such as flowers and fruits
They 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.
The plant kingdom has also 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 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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