Plant Systematics

Systematics (Lt. systema, organised whole) is the scientific study of the diversity of organisms, and any relationships between them. The idea coincides with taxonomy (Gr. taxis, arrangement; nomos, rule), which deals with the study of classification, including its principles, rules and procedures. Thus, plant systematics deals with the classification of the organisms recognised as plants.

In 1969, American biologist Robert H. Whittaker established a five-kingdom system recognising Monera, Protista, Fungi, Plantae and Animalia as distinct kingdoms. This classification is widely followed in textbooks.

Criteria for Classification

Members of the Kingdom Plantae are eukaryotic, multicellular, and mostly autotrophs that can perform photosynthesis. The plants are classified into various subgroups on the basis of-

Presence or absence of flowers
Presence of a differentiated plant body
Presence of seeds
Presence of a well-defined vascular system

Classification of Plants

On the basis of the mode of reproduction (seedless vs seed-bearing plants), the classification scheme of Kingdom Plantae is summarised as follows:

1. Cryptogams (Gr. kryptos, hidden; gamos, marriage)

Cryptogams are simple-structured organisms that do not make seeds and reproduce with the help of spores. Most of these plants or plant-like organisms lack a well-developed vascular system and a well-differentiated body with roots, stems or leaves.

A.1. Thallophyta (Gr. thallos, young undifferentiated shoot; phyta, plant)

Organisms with a thallus-shaped (undifferentiated), primitive and simple body.
Thallus may be branched or unbranched and colonial or filamentous in structure.
Lacks a well-developed vascular system.
Mostly autotrophic and can be found in moist places.

Example: Volvox, Spirogyra, Red algae, etc.

A.2. Bryophyta (Gr. bryon, moss; phyta, plant)

Organisms possess root-like, stem-like and leaf-like structures, but the body is still not properly differentiated.
Grow in the moist terrestrial region, but require water for reproduction.
Lacks a well-developed vascular system.

Example: Mosses, Liverworts, Hornworts, etc.

A.3. Pteridophyta (Gr. pteris, fern; phyta, plant)

Organisms with a well-differentiated body, divided into roots, stems and leaves- visible in the dominant sporophyte phase.
Large leaves called fronds are pinnately compound.
Possess a vascular system that helps in the conduction of water and minerals.

Example: Pteris, Adiantum, etc.

2. Phanerogams (Gr. phaneros, visible; gamos, marriage)

Phanerogams are considered higher plants that produce seeds for reproduction. Their body is well-developed with a highly developed vascular system and can be differentiated into roots, stems and leaves.

B.1. Gymnosperm (Gr. gymnos, naked; sperma, seed)

These plants possess a highly developed body that can be properly differentiated into roots, stems and leaves.
Well-developed tap root system present
Well-developed vascular system present.
Microsporophylls and megasporophylls aggregate to form male and female cones, respectively.
Fruits do not enclose seeds.
Do not possess flowers.

Example: Pinus, Cycas, etc.

B.2. Angiosperm (Gr. angeion, vessel; sperma, seed)

Plants characterised by flowers with double fertilisation
Fruits enclose the seeds.
Well-developed vascular system.

B.2.a. Monocot (Gr. monos, one; kotyledon, cup-shaped hollow)

Single cotyledon in the embryo.

Presence of an adventitious root; the tap root is temporary.

Cambium absent.

Parallel venation in leaves.

Example: Zea mays, Oryza sativa, etc.

B.2.b Dicot (Gr. di, two; kotyledon, cup-shaped hollow)

Two cotyledons in the embryo.
Presence of tap root.
Cambium is arranged in a ring.
Reticulate venation in leaves.

Example: Cicer arietinum, Mangifera indica, etc.

Importance of Plant Classification

Helps to understand the similarities and dissimilarities between the different plant groups.
Provides proper knowledge about the structure and life cycle of a particular plant.
Helps in the accurate identification and grouping of a particular plant.
Provides information about the primitive and the evolved plants, thus helping to understand the evolution of plants.

Frequently Asked Questions (FAQs)

Q1. Why are bryophytes referred to as the 'amphibians of the plant kingdom'?

Bryophytes grow in moist, terrestrial habitats, but during reproduction, the motile antherozoids (male gametes) need a film of water to move towards the female gamete. Thus, similar to the amphibians of the animal kingdom, bryophytes also depend on water for reproduction.

Q2. How can plants be classified on the basis of their life cycle?

On the basis of their life cycle, plants might be

Annual: Herbaceous plants completing their life cycle during a single season (corn, rice, pulses, etc.)

Biennial: Herbaceous plants that require two years to complete their life cycle (carrot, cabbage, beetroot, etc.)

Perennial: Woody or herbaceous plants that have a long lifespan, typically more than two years (lavender, dianthus, lilies, etc.)