A tree, botanically, is defined as a woody plant with numerous secondary branches supported clearly off the ground on a main stem (called trunk) and with conspicuous apical dominance. Trees are immortal, some living almost up to several thousand years; for example, the Great Basin Bristlecone Pine is the oldest tree in the world, and is 4844 years old! I'm sure it would have an interesting tale to tell, if it could speak! Most of the fruits we eat are borne on trees.
Need for Trees
From an environmental stand point, trees are the lungs of the Earth. Without trees, we'd all be dead. All photosynthetic plants produce oxygen, trees even more so. Apart from those in cultivation, trees in the wild contribute greatly to removing carbon dioxide from the air and supplying us with the vital gas, oxygen. All life depends on plants, more so on trees (especially wildlife).
Unlike a tree, a shrub is much shorter (never more than 8 m tall) and lacks a primary stem or trunk. Shrubs are usually quite messy, with many secondary branches. Shrubs are intermediate between herbs and trees, and can be cultured to develop into herbs or trees, depending on the cultivation conditions. Hardened shrubs may evolve into tree-like plants. However, they can still be identified as shrubs. Shrubs may be evergreen or may shed their leaves at maturity (deciduous).
Herbs are non-woody flowering plants. Commonly, a herb is a plant that is usually cultivated for economical gains by means of selling its parts (particularly flowers, leaves, seeds, and sometimes stem). Herbs are quite small in size (usually not more than several feet off the ground). Herbs have been largely cultivated by humans for their culinary, medicinal and / or cosmetic value and importance. Herbs can be commonly cultivated in small pots, as well as in the ground.
Many people prefer growing herbs for the use in different human aspects, and also because they do not require much space.
Vines, or creepers, are those plants that need a solid support for their growth. They are adapted to a crawling kind of growth habit. They lack a strong, stiff stem that will bear the weight of the plant. Majority of the creepers show adaptations that enable them to gain support by anchoring to a solid surface.
For example, many vines grow tendrils; these are curled structures that twirl around and firmly hold on to a solid vertical support and help the plant to grow. Some vines develop thorns on their ventral side; these cling onto rocks or walls and other similar textured or uneven surfaces.
Grasses are in a sense herbaceous plants; however, they cannot be classified as herbs. The main difference between herbs and grasses is that herbs have some kind of stem (though it is not stiff, sturdy or woody); however grasses completely lack stems. They might have a rhizome or bulb-like structure, but a true stem is absent.
Grasses have adapted to almost all kinds of habitats - from marshes to deserts. They are also widespread, in that they are found in almost every part of the world. Grasses can be very short (as in case of turfs) or very tall (as in case of food crops). Grasses are hence very versatile forms of vegetation.
Just as all life form depends heavily on trees, so does it on grasses. This is because grasses come at the base of the food pyramid. Grasses form primary food of many herbivores, including insects, rodents, deer, kangaroos, rabbits, cattle, sheep, horses, etc. We depend on most (if not all) these animals for something or the other. Grasses are hence very important.
These plants are adapted to the arid and hostile environment of a desert. These plants have the capability to store water. They also have the ability to use water efficiently. They usually have few or no leaves, which greatly reduces transpiration. Plant species like cacti and succulents, have dense flesh which is capable of storing large amounts of water. Several plants have developed the typical spikes and spines to prevent them from being eaten by animals.
They depend on other plants for support growing on trunks and branches. They are also dubbed 'air plants'. Mosses and orchids are instances of epiphytic plants. Though they are not parasitic, they can be deleterious to their hosts. They have an advantage over plants growing on the ground as they have a relatively better access to sunlight. They obtain nutrients from the air or water around them. They use photosynthesis for the process of nutrition.
Carnivorous plants normally grow in marshlands, bogs and areas of waterlogged, acidic and nitrogen poor soil (as in case of Pitcher plant); however, they may be found on grasslands as well (as in case of Purple Pitcher plant). carnivorous plants get their nitrogen intake by eating insects. The insect traps of a carnivorous plants use different attractants to invite prey. Other examples of carnivorous plants include Venus Flytrap, Utricularia, etc.
These plants grow in presence of abundant water. Aquatic plants have a lot of adaptations that enable the to survive the persistent wet conditions that they grow in. For example, the stems are unusually supple and bend easily with the flow of water, or the water current. The leaves have a thick waxy coating to prevent water from collecting. The plant is also quite buoyant (especially floating plants). Example of aquatic plants are hydra, water hyacinth, water lily, etc.
So that was a brief account of the different kinds of plants that exist in the world. It is said, however, that we have a record of only 1 in every 4 flowering plants; and flowering plants are not the only kind of plants that exist, as we just saw. So just imagine how many more plants there are out there that we have no clue about. Fascinating, isn't it?