In a majority of systems, which recognised only two kingdoms of organisms, the plants were included in the Plant Kingdom, whereas the animals were placed in the Animal Kingdom. For the classification of plants several systems were proposed from time to time. While using the number and position of stamens, Linnaeus divided flowering plants into 23 classes starting with the class Monandria with a single stamen (Canna) and plants with twenty or more stamens were assigned to class Icosandria. He also included all non-flowering plants such as algae, fungi, lichens, mosses and ferns in a separate class called Cryptogamia. His system was labelled as artificial system since it was based on a few characters. He himself stated that his system of classification did not consider the relationships. Moreover, his system places widely unrelated families of monocotyledons and dicotyledons in one class.
Subsequently, several systems of classification of plants were proposed by taxonomists. It is not possible to discuss all of them. We will describe a common system, which divides the plant kingdom into two sub-Kingdoms, named as Phanerogamae and Cryptogamae, considering the presence or absence of flowers and seeds. All flowering plants which bear seeds are included in Phanerogamae (Phaneros : visible; gamos: marriage), whereas the Cryptogamae (Kryptos: concealed; gamos : marriage) covers all non-flowing plants such as algae, fungi, lichens, mosses and ferns. The Cryptogams are further classified into Divisions - Thallophyta, Bryophyta and Pteridophyta each having classes assigned on the basis of similarities and differences among each group. The Phanerogams are also known as spermatophytes (sperma : seed : phyton : plant) since they produce seeds. These seed bearing plants are further grouped into two Divisions - Gymnospermae and Angiospermae. The gymnosperms (gymno: naked; sperma: seed) are represented by cycads, pines and cedars. These have naked ovules or seeds without any covering. The angiosperms include the entire flowering plants which produce seeds and have ovules enclosed in an ovary or fruit. Another important characteristic used for distinguishing phanerogams from the cryptogams is the presence of well-developed vascular tissues for conduction of water, minerals (xylem) and food (phloem), respectively. The plants having vascular tissues which resemble wind pipe of animals, are also called Tracheophytes (Trachea: wind pipe; phyton: plant).
You have learnt the distinguishing features of five kingdoms in the previous chapter. Here we will discuss the major plant groups, which were included in the plant kingdom and later assigned to kingdom Plantae of the Five Kingdom System. The groups described here include algae (other than blue-green), bryophytes, pteridophytes, gymnosperms, and angiosperms. Fungi are also excluded since it has been given the status of a separate Kingdom. Only the general characters of these groups along with the basis of their further classification are given. This will be followed by the classification system for flowering plants (angiosperms).