First level of classification among plants depends on whether the plant body possesses well-differentiated distinct cellular components or not.
Next level is based on the presence or absence of special vascular tissues for the transport of water and other substances. Further classification takes into account the ability to bear seeds and whether seeds are enclosed within the fruits or not.
Depending on these characteristics, kingdom-Plantae is divided as of Thallophyta, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms, as shown in the given flowchart:
The thallophytes, bryophytes and pteridophytes do not have external flower or seeds and have naked embryos called spores. These plants are therefore also called cryptogamae (meaning with hidden reproductive organs) because their reproductive organs are inconspicuous.
1. Thallophyta [Gk.thallos– undifferentiated; phyton -plant]
The characteristics possessed by thallophytes are as follows :
(i) They do not have well-differentiated body design.
(ii) Plants in this group are commonly known as algae.
(iii) These are predominantly aquatic, e.g. Spirogyra, Ulva, Chara, Ulothrix, Cladophora (green algae), Laminaria.
2. Bryophyta [Gk. bryon-moss; phyton-plant]
The characteristics possessed by bryophytes are as follows:
(i) These are called as the amphibians of the plant kingdom because they can live in soil, but are dependent on water for sexual reproduction.
(ii) In bryophytes, true vascular system is absent. They do not have specialised tissues for the conduction of water and food materials from one part of the body to another.
(iii) The body is commonly differentiated into stem and leaf-like structures. They lack real roots, stems, leaves, etc., and do not bear flowers.
Examples: It includes liverworts (Riccia, Marchantia) and mosses (Funaria).
3. Pteridophyta [Gk. Pteris– fern; phyton -plant]
The characteristics possessed by pteridophytes are as follows:
(i) These are the first land plants having vascular tissues.
(ii) The body of these plants is differentiated into roots, stems and leaves. They have specialised tissue for the conduction of water and food (well-developed vascular system, xylem and phloem). Therefore called as vascular cryptogams.
(iii) They do not bear flowers and do not produce seeds.
(iv) These include ferns (Dryopteris), club moss (Selaginella) and horse tails (Equisetum).
Plants which make seeds and have well-differentiated reproductive tissues are called phanerogams.
In these plants, seeds consist of embryo along with stored food. The stored food serves the purpose of initial growth of the embryo during germination. Seeds result from a reproductive process in plants. On the basis of whether the seeds are naked or enclosed in fruits, these are divided into two groups:
(i) Gymnosperms; and
(Gk. Gymnos -naked; sperma -seed)
The characteristics possessed by gymnosperms are as follows:
(i) These plants are the most primitive and bear seeds.
(ii) The seeds produced are naked and are not enclosed within the fruits.
(iii) Plants are usually perennial, evergreen and woody.
(iv) Sporophylls (spores bearing structures) are aggregated to form cones, which are separated into male and female cones, e.g. Cycas, Pinus (pine) and Cedrus (deodar).
[Gk. angion -covered; sperma -seed]
The characteristics possessed by angiosperms are as follows:
(i) These are also called flowering plants. Their seeds develop inside an organ called ovule. It modifies to become a fruit.
(ii) Plant embryos in seeds have structures called cotyledons. Cotyledons are also called seed leaves because in many cases they emerge and become green when the seed germinates.
The angiosperms are divided into two groups on the basis of a number of cotyledons present in the seed. The groups are:
(i) Dicotyledonous plants or Dicots
(i) Monocotyledonous plants or Monocots