Whittaker, a scientist, proposed a Five Kingdom classification for all living organisms dividing them into: Monera, Protista, Fungi, Animalia and Plantae. In Plantae, the species are subdivided into Algae, Bryophytes, Pteridophytes, Gymnosperms and Angiosperms. The classification of plants was done on the basis of their physical characteristics, called the artificial system.

Later, natural classifical systems included internal features like anatomy, phytochemistry etc, devised by George Bentham and Joseph Dalton Hooker. Currently, phylogenetic classification systems are used to classify plants based on the evolutionary relationship among organisms. Let us look into the world of kingdom Plantae: 

Algae

These are simple, autotrophic, aquatic organisms which can occur in a variety of habitats. They have variable sixes and can reproduce by vegetative, asexual and sexual methods. They are classified as:

Chlorophyceae: Usually, green algae are in this category, that are green due to the presence of chlorophyll a and b present in their chloroplasts. They possess a celluloid cell wall with an outer layer of pectose. They reproduce by fragmentation or different types of spores. If sexual reproduction occurs it happens by flagellated zoospores produced in zoosporangia. 

Phaeophyceae: The algae are brown and present in marine ecosystems ranging from simple, branched to filamentous forms.

Rhodophyceae: These are called red algae and owe their pigment to the presence of r-phycoerythrin in their body. They occur at the well-lit ocean surfaces and at great depths.They reproduce asexually by non-motile spores and sexually by non-motile gametes.

Bryophytes

These include mosses and liverworts which commonly grow in moist, shaded areas of the hills.These depend on water for reproduction but survive on land, hence, termed as amphibians of the plant kingdom. It lacks true roots, leaves and stem but possesses similar structures. Bryophytes have little to zero value for human beings but are a source of food for other herbivores. 

Pteridophytes

These include horsetails and ferns. They have separate vascular systems consisting of xylem and phloem. They are found in cool, damp places and have medicinal value. They also hold the soil together and help reduce soil erosion. They can be homosporous (having similar spores) heterosporous (having different spores). 

Gymnosperms 

The medium-sized trees, shrubs fall in this category. The special feature about these is that the seeds which develop after fertilization occurs are not enclosed in any wall or membrane. Their leaves are adaptive to the environment by growing in needle shape which covers a small area and have sunken stomata which helps them withstand hot, humid, dry climate. 

Angiosperms

All flowering plants are angiosperms as the male and female gametes develop in this enclosed structure and the process of fertilisation also takes place here. These occur in a wide variety of habitats and sizes. They have high economic value and are divided into the dicotyledons and the monocotyledons. 

Importance of plants:

Plants have developed before human beings existed. They are the only producers in the food chain. No other organism can make its own food from sunlight or any other source. They have ecologic, economic, and scientific value in regulating the temperature of the earth, by providing shelter to animal species and in studying the history of species, etc. It goes without saying, that we need to be more careful than our ancestors in protecting the plant cover as we do not have anywhere to survive.