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How many plant species are there?

    Today, scientists are aware of around 320,000-500,000 different species of plants, but they estimate this number to be much higher. Many of the known trees and flowers come from Asia, while other species of plants thrive in the tropical rain forests. Some plants have adapted to live in or below water, whereas some have adapted to live in the desert areas. Algae, lichen, and moss also belong to the plant kingdom.

 Lichen:
    Many lichens grow in inhospitable terrain; for instance, in the rocky highlands and cold Arctic. They also grow on the walls of the houses in big cities. The growth rate of lichens very slow -a few species grow less than one millimetre in a year. If you step on them, you may destroy in seconds a plant that took hundreds of years to grow. Since lichens are very sensitive to air pollution, they are often used as pollution indicators. The cleaner the air the more lichen will grow.
                                                                        
Algae:
    Algae are plant-like living organisms. They live in the seas, freshwater, or damp land areas. They range from tiny single-celled organisms, which can be  seen only under a microscope, to larger plant-like planktons, used as food by blur whales or flamingoes. Large leaf-like algae, known as 'seaweed', grow extensively in shallow waters to form dense underwater forests.

 Bacteria:
    Bacteria are single-cell organisms without the cell nucleus. Viruses are only nucleic acid molecules in a protein coat. They belong neither to the animals, which are multi-cellular organisms, nor to the plants, which always have a cell nucleus. While most viruses make humans, animals, and plants sick, many bacteria are good for human. For example, bacteria in the gastrointestinal tract help in digestion, whereas others like salmonella cause diseases.
How do we differentiate between plant species?
    Earlier the scientists differentiated between plants mainly by their appearance. Today, plants are classified according to their evolution, i.e., on the basis that all members of a group must have shared a common ancestor more recently than they have with any species outside the group. Thus, the plants are broadly classified as those containing chlorophyll (the green plants) and those without chlorophyll. The green plants include the green algae and the land plants, which grow towards light against the Earth's gravity. The land plants include the mosses, the vascular plants such as ferns and horsetails, and the seed plants. These also include the conifers (gymnosperms) and the flowering plants (angiosperms).
Difference between trees and shrubs? 
    The main difference between trees and shrubs is their size. Trees are much taller than shrubs and their root balls (system of roots with attached soil that supports the health of the tree) are also bigger. Unlike the thin and flexible stalks or stems of the shrubs, trees have thick stems to bear the load of the heavier and bigger leaves, branches, and fruits. Since trees are taller, they need to transport water from the soil much further up than the shrubs. This is done by several thin lair-like pipes in the stem and the branches.
    

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