We see a diversity of living organisms around us, plants, animals, fungi, viruses, and algae. But it’s important to know that the basic classification of living organisms includes two major kingdoms, Kingdom Plantae, and kingdom Animalia. Kingdom Animalia is heterotrophic organisms and kingdom Plantae are autotrophic in nature.  In this unit, we shall cover the characteristics and classification of the kingdom Plantae.

Characteristics of Kingdom Plantae:

Kingdom Plantae includes plants with the following characteristics :

1. These are multicellular organisms with properly arranged cell walls and frequently vacuolate eukaryotic cells.
2. They contain photosynthetic pigments in plastids.
3. Their principal mode of nutrition is photosynthesis but several plants have become absorptive.
4. Plants are primarily non-motile, living anchored to a substrate.
5. Structural differentiation leads towards organs of photosynthesis, anchorage, and support and in higher forms towards specialized photosynthetic, vascular, and covering tissues.
6. Reproduction is primarily a sexual or sometimes vegetative mode of reproduction.
7. Haploid generation is progressively reduced in higher members of the kingdom.

Classification of Kingdom Plantae:

The kingdom Plantae are multicellular eukaryotes with photosynthetic nutrition. Cells typically have cellulose walls, vacuoles, plastids, and several photosynthetic pigments which always include chlorophyll a. It can be broadly divided into

Cryptogams (plant without seeds): Algae (Thallophyte), Bryophytes, and Pteridophytes. Phanerogam (plant with seeds): Gymnosperm and Angiosperm.

The kingdom Plantae is classified into Algae, Bryophytes, Pteridophytes (seedless vascular plants), and Gymnosperms, Angiosperms.

Angiosperms are further divided into:

(i) Dicotyledons (with two cotyledons).

(ii) Monocotyledons (with one cotyledon).

Bryophyta, pteridophyte, gymnosperm and angiosperm are termed as the land plants or embryophytes. One major innovation of land plants was the evolution of the embryo. The embryo is defined as an immature sporophyte. Land plants can be vascular (or tracheophyte) and non-vascular. During the early evolution of land plants, nonvascular land plants diverged before the vascular plants. Non-vascular plants are often informally called bryophytes (from the Greek Bryon, moss, and python, plant). Bryophytes include liverworts, hornworts, and mosses. The terms bryophyte and bryophyta are not synonymous. Bryophyta is the formal taxonomic name for the phylum that

Classification System of Kingdom Plantae:

• The artificial system of classification used gross superficial morphological characters and was based mainly on vegetative characters or on the androecium structure.
• Natural classification systems were based on natural affinities among the organisms and consider not only the external features but also internal features.
• Phylogenetic classification systems are based on evolutionary relationships between various organisms which assumes that organisms belonging to the same taxa have a common ancestor.

The life cycle of Kingdom Plantae:

The sexual life cycle in plants alternates between diploid (sporophyte) and haploid (gametophyte) phases. Sporophyte literally means spore-plant, and gametophyte means gamete-plant. The haploid gametophytic body produces gametes by mitosis whereas the diploid sporophyte phase produces melospores (sexual spores) by the process of meiosis. The diploid phase produces the haploid phase by meiosis; the haploid gametes that fuse to make a zygote that starts another diploid phase. Three distinct versions of a generalized sexual life cycle occur among plants:

Different plant groups complete their life cycles in different patterns. Angiosperms complete their life cycle in two phases- diploid sporophytes and haploid gametophytes. The two follow each other. This phenomenon is called alternation of generation.

• Haplontic- Saprophytic generation is represented by only the one-celled zygote. Meiosis in the zygote results in haploid spores forming gametophytes, which is the dominant vegetative phase. Example- Volvox, Spirogyra, etc.
• Diplontic- Diploid sporophytes are dominant, independent, photosynthetic plants. The gametophyte is represented by single to few cells. All seed-bearing plants fall under this category.
• Haplo-diplontic- Both phases are multicellular and an intermediate condition is present. It is present in Bryophytes and Pteridophytes.

Conclusion:

Therefore, in this unit, we study the Plant Kingdom. It comprises multicellular, photosynthetic organisms constituting the predominant producers of the biosphere. They may be aquatic, terrestrial, or may even occupy other habitats. They comprise hundreds to thousands of different species. Owing to this vast diversity and number of species, various classification systems have been formulated from time to time. For example, Linnaeus (1754) divided the Plant Kingdom into 25 classes on the basis of a number of stamens and carpels, their union, and their presence or absence in a flower.