Plants are an incredibly important kingdom of organisms. They are multicellular organisms with the amazing ability to make their own food from carbon dioxide in the atmosphere. They provide the foundation of many food webs and animal life would not exist if plants were not around. The study of plants is known as botany and in this introduction to plants we look at key topics such as the process of photosynthesis, different types of plants and the different parts of a plant such as roots, stems and leaves.
By using the sun’s energy to fix carbon dioxide, plants are able to produce sugars through a process known as photosynthesis. The sugars produced through photosynthesis provide plants with the energy to survive, grow and reproduce. As plants grow they become a food source for animals and other organisms.
Other woody plants include a group known as the gymnosperms. This group includes pine trees and their relatives plus other non-flowering trees. Less advanced plants include ferns, lycophytes and mosses.
Plants made the move from water to land around 500 million years ago. Living on land is significantly different to living on water and plants have had to make serious changes to their body plans in order survive on land.
Land plants separated their body plans into roots, stems and leaves. Roots absorb water and nutrients from soil, stems transfer materials between roots and leaves, and leaves produce sugars that provide the plant with energy to survive.
Photosynthesis is a key topic for an introduction to plant biology. It is a process that occurs in plant cells that uses the sun’s energy to produce sugars from carbon dioxide and water. The process is simply a series of chemical reactions, probably the most important chemical reactions of Earth.
The green color of plants is caused by a molecule called chlorophyll a. Chlorophyll a has the ability to absorb light energy from the sun. The energy that is absorbed is used to force reactions with water and carbon dioxide. The result of these reactions is the production sugars and oxygen gas.
The overall reaction looks like this:
energy + water + carbon dioxide → sugar + oxygen
Photosynthesis takes carbon dioxide from the atmosphere, converts it into sugar and releases oxygen back into the atmosphere. Over time photosynthesis changed the atmosphere of the Earth by increasing the amount of oxygen in the air.
Vascular vs. non-vascular
A critical step in the evolution of current plant species was the evolution of vascular tissue. Like humans have vascular tissue that transports blood through our bodies, the majority of species of plants have vascular tissue that transports water and nutrients around their bodies.
Before plants evolved vascular tissue, water was only able to enter into a plant by diffusing through the plant’s cells. This meant plants were unable to grow very large because diffusion is not efficient enough to support large plants. Once plants evolved vascular tissue, they were able to grow much larger and which allowed the evolution of the giant trees that now grace the Earth’s lands.
There are still many species on non-vascular plants but the vast majority of plant species contain vascular tissue. Non-vascular plants include organisms such as mosses and liverworts. Some biologists also consider the green algae to be non-vascular plants. Because non-vascular plants rely on diffusion to absorb water they are typically found in moist environments.
Vascular plants make up over 90% of all plant species that are currently found on Earth. More primitive vascular plants include lycophytes and ferns. These two groups reproduce with spores rather than seeds and are unable to produce wood.
Gymnosperms and angiosperms are the two most recently evolved groups of vascular plants. They can both produce wood and reproduce with seeds rather than spores.
Plant body plan
Plants have a relatively simple body plan. A plant can be split into two sections: the underground system known as roots and the above ground system referred to as shoots. The shoots typically include stems, branches and leaves.
The evolution of roots was key to the success of plants on land. Roots grow underground in search for water and nutrients in the soil. Often almost half of a plant’s mass is hidden underground in the root system.
Roots also help to anchor a plant to the ground so it doesn’t get blown away in the wind or in a flood. They can also be used to store excess food to be used at a later date.
Stems and branches connect leaves and roots to each other. They are the ‘highways’ that water, nutrients, and sugars travel through to nourish the various parts of a plant.
Branches and stems influence the height and size of a plant which in turn affects how much light it will receive from the sun. A stem and branch can be green and fleshy but in many plants, they are brown, woody and covered in bark.
Leaves are the main place where photosynthesis occurs. The leaves of the plant have the responsibility of producing enough energy to feed the entire plant. Leaves are optimized for this challenge.
A typical leaf is full of a green molecule called chlorophyll a which is the magic ingredient in photosynthesis. Chlorophyll a is able to use energy from the sun to kick start the process of photosynthesis. Leaves are also usually flat and have large surface areas to capture as much light from the sun as possible.
An angiosperm is any plant that produces flowers, fruit, and seeds. They are the most advanced, diverse and abundant group of plants. Angiosperms include the majority of the plants that most people are familiar with such as grasses, orchids, roses, lavender, magnolias, plus the plants that produce the fruits, vegetables, grains and nuts that we regularly eat.
Flowers and fruit evolved as a part of a plant’s reproduction. Flowers produce pollen and an ovary. Pollen from one flower is delivered to the ovary of another flower – this is known as pollination. A sperm cell found in a pollen grain fertilizes an egg located in an ovary. Once the egg is fertilized, it develops into a seed and the ovary develops into a fruit.
Angiosperms have a close relationship with animals, in particular insects and birds. The pollination of flowers is most commonly assisted by animals. Flowers provide animals with nutritious foods such as nectar and pollen.
As animals move between flowers feeding on nectar and pollen, they transfer pollen between flowers. The newly delivered pollen then has the ability to fertilize the egg of the new flower.
Many angiosperms and animals have evolved alongside each other and their survival depends on one another. If the angiosperm goes extinct, the animal loses its food source. If the animal goes extinct, the plant loses it pollinator and cannot reproduce.
Gymnosperms are the closest relatives of angiosperms. They are a group of woody plants that produce seeds but no flowers or fruit. The seeds of gymnosperms are usually found in cones rather than inside fruit.
The world’s largest, tallest, oldest and widest organisms are all gymnosperms. They are incredible plants and some species are known to survive for over 2000 years.
Ferns and lycophytes
Ferns and lycophytes are non-woody plants and also don’t produce seeds, flowers or fruit. Instead, ferns and lycophytes reproduce using tiny structures called spores.
These two groups were once the most common plants of Earth but they have since been outgrown by gymnosperms and angiosperms. Still, around 12,000 species of ferns and 1,200 species of lycophytes remain on Earth.
The main difference between ferns and lycophytes is in the vascular tissue of their leaves. Ferns have fronds with multiple veins whereas the leaves of lycophytes are very simple and only have one vein.
Besides lacking tissue, non-vascular plants also lack wood, roots and flowers. This group of often ignored plants includes mosses, hornworts, liverworts and (depending on who you’re talking to) some algae. Compared to vascular plants, non-vascular plants are small and they struggle to grow taller than a few centimeters.
Mosses are the most common and best known of the non-vascular plants. They include over 14,000 species that are found all around the world.
Liverworts and hornworts are two groups of underappreciated plants. They are flattened plants that are typically only a few millimeters tall but grow sprawling across moist surfaces and are commonly mistaken for mosses and algae.
Video by Frank Gregorio. To see more brilliant videos like this one, check out Greg’s vimeo channel
For a deeper look at the biology of plants check out the plant section of our website:
Plants – Basic Biology
Last edited: 16 December 2016