Plants are beings multicellular and eukaryotes. In these respects they are similar to animals and many types of fungi; however, they have a feature that distinguishes them from these beings - are autotrophic. As we have seen, autotrophic beings are those who produce their own food through the process of photosynthesis.
Using light, that is, light energy, plants produce glucose, organic matter formed from water and carbon dioxide they get from food, and release oxygen gas.
Plants, along with other photosynthesizing beings, are producers of organic matter that nourishes most of Earth's living beings, acting at the base of food chains. By providing the oxygen gas In the environment, plants also contribute to the maintenance of the life of beings who, like themselves, use this gas in respiration. Plants conquered almost every environment on the earth's surface.
According to the most accepted hypothesis, they evolved from protist ancestors. Probably, these ancestors would be types of algae belonging to the group of protists that developed in the water. Similarities were observed between some types of chlorophyll that exist in both green algae and plants.
From these and similarities, it is assumed that aquatic green algae are direct ancestors of plants.
About 500 million years ago, plants began occupying the terrestrial environment. This environment offers plants advantages such as easier light capture as it does not reach deep water depths and easy gas exchange due to the higher concentration of carbon dioxide and oxygen gas in the atmosphere. These factors are important in the process of respiration and photosynthesis.
But what about the presence of water, so necessary for life?
Comparing the terrestrial environment with the aquatic environment, we find that in terrestrial the amount of water in liquid form is much smaller and also that most of it is accumulated inside the soil.
How, then, do plants survive in the terrestrial environment? This is possible because they present adaptations that enable them to develop in the terrestrial environment and occupy it efficiently. Plants adapted to the terrestrial environment have, for example, structures that allow the absorption of water present in the soil and other structures that prevent excessive loss of water. We will see later how this occurs.
We must remember that some groups of plants continued to survive in the aquatic environment.
Plants cover much of the earth's environments on the planet. Taken together, as in this photo, they all look the same. But in reality there are several types of plants and they occupy the most diverse environments.
You already know that to classify, that is, to organize various objects or beings into different groups, we must determine the criteria by which we will identify the similarities and differences between them.
Let's see now how plants can be classified.
The kingdom of plants is made up of organisms multicellular cells, eukaryotes, photosynthesizing autotrophs.
It is necessary to define other criteria that enable the classification of plants to organize them into groups less comprehensive than the kingdom.
In general, scientists consider as important criteria:
- the characteristic of the plant to be vascular or avascular, that is, the presence or absence of conducting vessels of water and mineral salts (raw sap) and organic matter (elaborate sap);
- have or not reproductive structures (seed, fruit and flower) or absence of them.
Plant group names
- Cryptogam: word composed by crypto, which means hidden, and gamma, whose meaning is related to gamete (reproductive structure). This word therefore means "plant that has hidden reproductive structure". That is, without seed.
- Phanerogamous: word composed by fanero, which means visible, and for gamma, relative to gamete. This word therefore means "plant that has visible reproductive structure". They are plants that have seed.
- Gymnosperm: word composed by gimmno, which means discovery, and sperma, seed. This word therefore means "bare seeded plant" or "bare seed".
- Angiosperm: word composed by angion, which means vase (which in this case is the fruit) and sperma, seed. The word means, "seed plant kept within the fruit."