|Plants form one of the
two big groups into which all living things can be classified.
These are different from the other big group, called animal, in so many ways. Yet, these differences do not apply for all variety. Neither for plants, nor for animals. Because there are many living beings whose structures are so simple that they are difficult to fit one of the two big groups.
The cell is the building block from which all living things are made. The typical plant cell has a cell wall that contains the chemical called cellulose. This material strengthens the plant wall and gives plants their relatively rigid form. Cellulose is almost entirely absent in the animal kingdom. Most plants are stationary. They grow in one place. And they die in the same place. Whatever movements they have, are limited to changes brought about by growth.
The plant kingdom consists of approximately 350,000 kinds of plants. They range in size from too small to be seen with naked eyes, the bacteria type, to the 350 feet high, giant Redwood trees.
The plants have come up earlier than the animals appeared on this planet. Since then they have been evolving for millions of years. While they have started with the simplest form they keep on adding to the complexity of their forms over thousands of years. Based on the time of their arrival, the plant kingdom has been divided into different groups. The most recent type is the flowering plant. Before this all were of non-flowering variety. And we all come to know of them by studying the prints of their remains preserved in ancient stones and mineral deposits. These are known as fossils and help us studing the past.
Studying these fossils we come to know that the first and the foremost of them all are the tiny algae and bacteria. In fact, these are also the first form of life. And hence, the simplest ones to inhabit Earth. They all came up in the Pre-Cambrian Era. Though these plants lacked chlorophyll, they could create their own food from nonliving or, inorganic substances.
One of those simple forms are the blue green algae. They are one celled and that too is not so well defined as are in other plants and animals. Yet, they are plants. For, they manufacture their own food from carbon dioxide and water, using the energy of the sun. There were
various other types of primitive algae. The brown algae, red algae, green algae, the yellow green algae, diatoms, euglenoids, are all some of these primitive variety.
While the algae, like the blue green ones, are still very much common, another variety evolved with some well defined cell structures. And they could move, with the help of an organ, called flagella. But they are not plants. Neither were they animals. However, they had some features of the both. And scientists believe that it is from these flagellates, more developed plants and the first form of animals evolved. Diatoms are also a kind of blue green algae found in various shapes and sizes. Some move while others do not. They occur in fresh water as well as in the soil. But they most abundant in the oceans and bays. Living diatoms store part of their food
as oil. Thus they were probably one of the important contributors to the Earth' great deposits of petroleum. When dead their soft parts decay, leaving the almost see-through glassy cell wall to be deposited at the soil.
The algae apart, there were other forms of life as well. The fungus, the bacteria and the moulds, all belonging to the primitive form of plant lives. Few fossils of the fungi exist. The mycelium of some parasitic fungi have been found in fossil wood and the fruiting bodies of some of the larger variety of fungi have been found attached to fossil tree trunks. Most of the scientists believe that the simplest of the fungi developed from early flagellates that had lost their chlorophyll. From these simple fungi, came up the ascus or sac fungi. And from this group the basidium fungi developed. Some of the fungi are poisonous. Some other are, however, edible. But many are too woody or hard to eat.
The origin of the slime moulds is less well understood. Some belive they were modified from the one celled amoebas, and hence, are more nearly animals than plants. Probably it was the green algae that gave rise to higher plants. For instance, the plants, like, the liverworts, hornworts and mosses. Many scientists now believe that the hornworts changed over millions of years to form the vascular plants. These plants have special conducting tube like tissues, and hence the name. These tissues help transport the raw materials (pre cooked) and the prepared food (cooked), from one part of the plant to another. Vascular plants that do not produce seeds are called the lower vascular plants. There are four main groups: Psilotum-like plants, club mosses and quillworts, horsetails and scouring rushes, and ferns. Except for the first variety, all of these plants have roots, stems and leaves. And they reproduce by spores and have an alternation of generations.
Psilopsids are the most primitive of the lower vascular plants. They flourished during the Devonian times, some 300 to 400 million years ago. They were the first plants to conquer
land. Today only two of this variety survive. These are Psilotums, like the whisk broom fern, found widely in the tropics. And Tmesipteris, found in the Philippines and also in New Zealand.
Club mosses, the close relative of Psilotums, also belong to the vascular plants. In it the vascular system is more advanced. For, the conducting tubes age grouped into bundles and are distributed through the stem.
In the Coal Age, 230 to 260 million years ago, ferns and their kin were the trees of the swamp forests. Many grew more than 100 feet high. Coal was formed from the remains of these giant ferns and related plants. Some ferns, though still ranked as lower vascular plants, have woody stems in which the conducting tubes of the vascular system has began to be organized into definite rings, comparable to the arrangement in higher plants.
Non flowering seeds plants come next to the vascular variety. These are placed in the higher rung of the plant pyramid, arranged from the simplest algae at the base to the most recent flowering variety at the apex. And none of the plants in its lower rungs, including all algae, mosses, liverworts, ferns and club mosses, do not produce seeds.
Development of seeds was an important advance in plant evolution. A hard outer coat
protects a seed from dying out. It also protects injurious ultraviolet rays of the sun and also from the fungi infection. Seeds can remain dormant for long periods until conditions are just right for their growth. Inside the seed is the embryo, a young plant. Food stored in the seed enables the embryo to live and grow until it can manufacture its own food by photosynthesis. Seeds permit young plants to adapt to a far greater variety of growing areas and under a greater variety of conditions than do the spores.
Again seeded plants can be divided into two groups. The one that does not produce flower , but has seeds. And the one that flowers and bears seeds as well. The former ones are called gymnosperms, or, naked seed plant. This variety include pines, firs, cedars, cypress and spruces, as well as cycads and gingko. They are found in every part of the world. But they are more abundant in the temperate regions than in the tropics. The great evergreen forests of Canada and Alaska. And they also grow in the cool temperate regions of Europe and Asia.
All of them are mainly conifers or, have the cones and needle like or scale like leaves.
Sequoias and Redwoods, the two largest living trees, belong to this group. Some of them are 3000 to 4000 years old. The bristle cone pines that grow high on the mountain slopes in the White Mountains of California are even more ancient, some of them estimated to be more than 4,500 years old.
Some 230 million years ago these conifers were the dominant plant group on Earth. But they have declined since Mesozoic times, while the angiosperms or flowering plants have become more important.
At the top most rung of the plant pyramid rest the angiosperms or the flowering plants. These plants are also of latest origin, coming up around 150 million years during the lower cretaceous period. They evolved from gymnosperms. However, it is not yet certain from which group of gymnosperms they originated. The basic difference between gymnosperms and angiosperms is that latter one reproduce by means of flowers. Flowers are modified shoots bearing a series of leaflike forms which containing immature seeds. Along with other features, the enclosed condition of the seed gave the flowering plants an advantage and enabled them to come to dominate the world of plants. Flowering plants have also fully exploited the use of insects and other animals as agents of their reproduction. In addition, the water-conducting cells and food-conducting tissue are more complex and efficient in flowering plants than in other land plants. Finally, flowering plants possess a specialized type of nutritive tissue in the seed, endosperm. Endosperm is the chief storage tissue in the seeds of grasses; hence, it is the primary source of nutrition in corn, rice, wheat, and other cereals that have been utilized as major food sources by humans and other animals.