There are nearly 3, 92,000 species of flowering and non-flowering plants which have been identified.
Depicts details on the total number of plant species recorded so far and the number believed to exist on earth. The number of plant species that occur in India is more than 45,000, which represent nearly 12 percent of recorded flora of the world.
The huge number of plant species inhabiting the earth show great diversity with respect to their habit, habitat, structure, function and life-span.
They range in size from microscopic bacteria, some of which are some thousandth of cm in diameter, to giant Sequoias which grow to more than loom, in height and may weigh more than 1000 tons. A giant Californian Sequoia may probably be more than 3500 years old. There are certain coniferous plants which are more than several hundred years old.
Plant form varies with varying environmental conditions. Plants that grow in dry areas and swampy areas vary distinctly. The same comparison can be made between plants found in tropical and polar climates.
The soil factors, wind conditions and the duration of sunlight also determine the forms of vegetation. For example, the sandy soil of coastal plains does not hold water very well. Thus, it can support plants whose leaves are much reduced or modified. But the broad-leaved trees grow better in the soil with more humus.
The origin of plants continues to be a debatable question. Fossil data indicate that the plants have originated nearly 2 billion years back.
In this long span of time, many plants have originated, many more evolved into complex, better adapted ones and still many unknown number who failed to evolve with time have become extinct. At present the large numbers of plants that surround us are the products of this evolutionary process.
The most primitive and the smallest plants in terms of their structural simplicity are the bacteria and algae. Bacteria are mainly known for the diseases they cause to humans and role played in decay and decomposition of organic materials.
The algae, on the other hand, are the simplest photosynthetic plants. However, the term algae do not refer to a single group of plants but to a multitude of its representatives who vary primarily on the basis of pigmentation, complexity of form and elaboration of process of reproduction.
Fungi like the algae are primitive on the scale of plant evolution. These are however, non chlorophylls plants and live as parasites or saprophytic ally on dead, decaying organic debris.
They seem to have an evolutionary development parallel with algae. Lichen is a dual organism, where an association is formed between algae and fungi. Some fungi are synonymous with plant diseases yet others are important in baking industry, alcohol production and in manufacture of antibiotics.
The plants discussed so far, are really numerous and diverse. But the common man is not familiar with these small inconspicuous diverse worlds of plants. To him the term plants refer to some familiar, green leafy land habiting organisms. However, land plants evolved considerably later in comparison to aquatic algae.
During this transition from water to land, it appears that evolution has occurred in two distinct lines. One of these was appearance of conducting or vascular tissues. These plants reached great degree of diversity in form and function. In second line, no specialized vascular or supporting tissues developed and these plants gave rise to no other new forms. Modern descendants of the latter are liverworts, hornworts and mosses. These are collectively called as bryophytes.
These are most primitive of green land plants, predominantly amphibious in habit. Individually, the bryophytes are small inconspicuous and often seen growing in clusters. The vascular plants which represent another line of evolution from the primitive aquatic plants are more than 400 million year old. Club mosses, horsetails, ferns form the conspicuous representative of this type of organisms.
In course of time, a few vascular plants started to produce seeds. Among the earliest seed plants are the pines, cycads, fir trees etc. collectively called gymnosperms? Following these, a special type of flowering plants made its appearance.
These are closed seeded; they are now the dominating forms of plant life. They are not only the highest forms of plant life, but the most diversified and widespread, as well.
A brief survey of plant kingdom shows that some plants may lack roots, stems and leaves, others are non green, some do no, contain the supporting and conducting system Some do not form seeds, other have naked seeds and some plants have flowers, from winch.
Seeds with integument, called fruits develop. Again, the world of plants shows a great diversity in their life cycle pattern. The simplest and earliest forms have haploid plant bodies called thalli, i.e. the gametophyte or haploid or n phase is quite evident in the life cycle. However, saprophyte or diploid or 2n phase is only restricted to zygote or the fusion products of sexual reproductive unit.
These diploid or saprophytic or zygotic phase is very Short lived and never becomes a free living plant at maturity. It undergoes meiosis or reduction division to produce the haploid, free living and independent phase.
When the life cycle pattern of bryophytes are considered, one will find that the plant body is more complex foliose and haploid. However, bryophytes along with saprophytic generation are never an independent, free living plant at maturity. In contrast, the pteridophytes are diploid or saprophytic generation is more prominent and independent.
In the flowering plants like gymnosperms and angiosperms, the saprophytic or diploid (2n) generation reached its zenith of elaboration and gametophytic or haploid (n) generation is parasitic upon it.
They are in the form of gametes. The condition is just the reverse of the pattern of life cycle seen among the thallophytic like algae of fungi. Among the individual divisions of plants like algae or fungi, bryophytes, pteridophytes etc. great degree of diversity in life cycle pattern is also observed.