Plants: the ultimate guide. Information, pictures and facts on plants, why they’re important, the different types of plants and how plants reproduce.
Plants: The Ultimate Guide
On this page you’ll find a complete guide to plants and the plant kingdom. You’ll discover what makes a plant a plant, why plants are important, the different types of plant and how plants reproduce.
This is the first in a series of articles on the plant kingdom. Over the following weeks we’ll be adding additional pages covering subjects such as plant reproduction, classification, the parts of a plant and practical botany in greater depth.
Plants: Page Index
- Why Are Plants Important?
- What Is Botany
- What Is A Plant?
- Characteristics Of Plants
- Is Algae A Plant?
- How Many Species Of Plant Are There?
- Types Of Plant
- Land Plants (Embryophyta)
- Bryophytes (Non-Vascular Land Plants)
- Vascular Plants
- Vascular Plants That Don’t Produce Seeds
- Seed Plants
- How Do Plants Reproduce?
- Further Reading
If you take a look out of your window, you’ll almost certainly be able to see some form of plant life. Whether it’s weeds growing in a backyard or trees in a forest, there are very few landscapes on Earth in which plants are not present.
Without plants, the world would be a much less-colorful place. But plants provide much more than just decoration; plants are vital for life on Earth.
Why Are Plants Important?
Plants are important because, unlike most other organisms, nearly all plants are autotrophs, which means that they produce their own food. Autotrophs don’t have to eat other organisms (i.e. other plants or animals) for energy.
Most plants are capable of making their own food using a process called photosynthesis, in which energy from sunlight is transformed into chemical energy.
All other organisms (including animals) are reliant on the energy produced by autotrophs.
Plants are the primary producers in virtually all terrestrial (land-based) food chains. Even carnivorous animals such as tigers are reliant on the energy provided by plants – even if they get it second-hand, by eating animals that do eat plants!
Another reason that plants are important is that they produce much of the world’s oxygen. Plants produce oxygen as a by-product of photosynthesis.
In addition, plants help to reduce the amount of carbon dioxide in the atmosphere. This too is a result of photosynthesis. It has been calculated that forests in the U.S. offset between 10 and 20% of the nation’s annual emissions. (Source)
As well as relying on plants to supply our food and much of our oxygen, we also use plants for fuel, building materials, clothing, medicines, cosmetics, and a host of other things that make our lives easier.
What Is Botany?
Botany is the branch of biology concerned with plants and plant life (biology itself is the scientific study of life and living things). A scientist who specializes in botany is known as a botanist.
What Is A Plant?
A plant is a living organism in the kingdom Plantae. Plantae is one of the five (or six, depending on which system is being used) kingdoms into which all living things are categorized.
Other kingdoms include Animalia (the animal kingdom) and Fungi (fungi aren’t plants).
- You can find out more about kingdoms and how living things are categorized on this page: Animal Classification
Some Characteristics Of Plants
In order to be included in Plantae, an organism must have certain characteristics. Some basic characteristics of plants are listed below:
Plants are eukaryotes
Plants are eukaryotes, which means that their bodies are made of eukaryotic cells. Eukaryotic cells contain a nucleus and other organelles, each of which is enclosed within its own membrane.
Plants aren’t the only eukaryotic organisms. Other eukaryotes include animals and algae. All multicellular organisms are eukaryotes.
Organisms whose cells are not eukaryotic are known as prokaryotes. Prokaryotes are single-celled organisms such as Archaea and Bacteria. The components of a prokaryotic cell are not enclosed in membranes.
(Most) plants photosynthesize
As we’ve found, most plants are autotrophs. They produce their own food via photosynthesis, a process in which energy from the sun is converted into chemical energy in the form of carbohydrates.
Plant cells contain organelles known as chloroplasts in which photosynthesis takes place.
Not all plants photosynthesize; a small number of plant species are parasitic, obtaining energy and nutrients from other plants rather than from sunlight.
Plants aren’t the only organisms capable of photosynthesis; organisms such as red algae and cyanobacteria are also photosynthetic.
Plants contain chlorophyll
Chloroplasts contain chlorophyll, a pigment that absorbs the sunlight that allows photosynthesis to take place.
Plants appear green because of the chlorophyll contained in their cells. Chlorophyll absorbs blue and red light. Green light is reflected, making the plant look green to our eyes.
Plants contain two types of chlorophyll: chlorophyll a and chlorophyll b. It’s the presence of both types of chlorophyll that distinguishes plants from other photosynthesizing organisms, such as cyanobacteria and red algae.
Some parasitic plants have lost the need to photosynthesize. Plants such as these don’t contain chlorophyll and are therefore not green.
Plants store starch
Starch is a polysaccharide (a long chain of carbohydrate molecules) which the plant uses as a source of energy and matter. Plants store starch in organelles in their cells called plastids
The cell walls of plants contain cellulose
Cellulose is a polysaccharide. It forms strengthening fibers in the cell wall of a plant cell.
All land plants are multicellular
All land plants (and aquatic plants that are not algae) are multicellular. Some green algae (see below) are unicellular in at least part of their life cycle.
Is Algae A Plant?
Historically, the kingdom Plantae contained only the land plants, a group of organisms known as Embryophyta. Algae, which are photosynthesizing organisms found mainly in water, were not included in Plantae.
It is now thought that the ancestors of plants arose from a group of algae known as green algae. This means that the land plants share a common ancestor with green algae.
As a result of this new understanding, the definition of plants is today usually expanded to include green algae. This expanded group may also be known as the ‘green plants’, or Viridiplantae.
Other types of algae (such as red algae) are not usually included in Plantae.
How Many Species Of Plant Are There?
Estimates of the total number of living plant species range from 320,000 to 382,000 living species. (Source)
While this number may be large, it is significantly smaller than the number of living animal species (of which there are an estimated 1,525,728).
Although it contains fewer species than Animalia, Plantae is by no means a small group: there are over twice the number of living plant species than there are of fungi (there are an estimated 140,000 living species of fungi).
- You can find out more about photosynthesis on this page: Photosynthesis Facts
Photosynthesis is the process by which a plant transforms energy from sunlight into chemical energy. Photosynthesis takes place in the chloroplasts present in the plant’s cells.
Chlorophyll within the chloroplasts absorbs the sunlight that provides the energy required by photosynthesis.
This energy fuels a chemical reaction in which carbon dioxide from the air is combined with water in order to produce glucose (a simple carbohydrate). As a by-product of this chemical reaction oxygen is produced.
The plant obtains energy from the glucose via a process known as respiration. The plant uses this energy for various functions, not least for making the cellulose that strengthens the plant’s cell walls.
Any glucose that isn’t required immediately is converted into starch and stored in the plant’s chloroplasts for use when the plant isn’t able to photosynthesize (i.e. during the winter).
Types Of Plant
- A list of the major plant groups, with pictures and information, can be found on this page: Types Of Plant
Just like animals, plants are divided into groups depending on their family tree. First the entire kingdom Plantae is divided into large groups, and these groups are then divided into ever-smaller groups of ever-more closely-related species.
We found above that green algae, which are mainly aquatic, are now considered to be part of the plant kingdom. This is because they share the same ancestors as the other main group of plants, the land plants, or Embryophyta.
Land Plants (Embryophyta)
The land plants, or embryophytes, are what was traditionally thought of as ‘plants’, before the green algae were included. Most embryophytes live on land, although some, such as lilies, duckweeds and bulrushes, are partly or entirely aquatic.
There are two main types of land plants: vascular plants and bryophytes. There are over 300,000 species of vascular plants and around 20,000 species of bryophytes.
Bryophytes (Non-Vascular Land Plants)
Mosses, hornworts and liverworts
Non-vascular land plants form a group known as bryophytes. The group includes the mosses, hornworts and liverworts. The plants in this group are thought to have descended from the earliest land plants. They do not produce flowers or seeds, and reproduce using spores.
Spores are single reproductive cells that, under the right conditions, will germinate and grow to become a new individual. Bryophyte spores grow into new plants by making exact copies of their cells – a process known as mitosis.
Plants aren’t the only organisms to reproduce using spores; fungi, algae, and some unicellular organisms also produce spores.
Bryophytes are usually small, and found in moist, shady habitats. They have single-celled root structures and are not dependent on their roots to collect nutrients (for this reason they can live on rocks).
Bryophytes are very sensitive to air pollution, and tend not to be found in highly polluted areas.
Vascular plants belong to the group Tracheophyta, and are known as tracheophytes. Plants in this group contain vascular tissue, which carries water and nutrients around the plant.
Vascular tissue itself contains two different types of tissue:
- Xylem transports water and minerals from the plant’s roots around the plant;
- Phloem transports sugar produced by the plant’s leaves around the plant.
Situated between the xylem and phloem is a band of cells called the cambium.
Vascular plants are divided into two main groups: those that produce seeds, and those that don’t.
Vascular Plants That Don’t Produce Seeds
Vascular plants that don’t produce seeds are known as pteridophytes. Pteridophytes lack flowers and use spores to reproduce.
This group of vascular plants includes ferns and horsetails. The term pteridophytes is often expanded to include the clubmosses (lycophytes), although these plants are less-closely related and are often placed in their own group.
With over 10,000 species, the ferns are the largest group of non-seed producing vascular plants.
Seed plants are also known as spermatophytes. As the name suggests, seed plants grow from seeds rather than from spores.
There are two main groups of seed plant: gymnosperms, many of which grow their seeds in cones; and angiosperms, which are flowering plants whose seeds are protected and dispersed within fruit.
Unlike spores, seeds are multicellular structures (i.e. they consist of more than one cell). A seed not only protects the plant embryo, but also feeds it.
A plant seed consists of three parts: the seed coat is a protective covering; the endosperm is the tissue which provides nutrients; and the plant embryo is the part that develops into the new plant.
Under the right conditions, the seed will germinate (begin to grow). Until a seed germinates it is said to be dormant. Seeds don’t always germinate immediately they are dispersed; they can be dormant for periods of weeks or even many years.
A growing seed is known as a seedling. It lives off the nutrients contained within its seed until its leaves are sufficiently developed to allow it to photosynthesize.
Conifers, cycads, ginkgo, and gnetae
Gymnosperms are seed plants whose seeds are not contained within a fruit. The word gymnosperm means ‘naked seed’, and refers to the exposed nature of the seeds produced by members of the group.
The seeds of many gymnosperms are produced in cones.
The most diverse and best-known gymnosperms are the conifers. Familiar trees such as pines, firs, cedars, spruces and redwoods are all conifers.
The other three groups of gymnosperms are the cycads, gnetae (plants in the division Gnetophyta), and the ginkgo (Ginkgo biloba is the only surviving ginkgo species).
Angiosperms are planting whose reproductive parts are contained in flowers. The seeds of flowering plants are protected and dispersed within a fruit.
Flowering plants first appeared in the Cretaceous Period. They diversified rapidly, coevolving with bees and other pollinating insects.
With over a quarter of a million known species, the flowering plants are easily the most diverse of all plant groups. Well-known angiosperms include roses, daisies, grasses, heathers, cacti, palms, and trees such as poplars, birches, beeches and elms.
Monocots and Dicots
There are two main types of angiosperms: monocots and dicots.
Monocots are also known as monocotyledons. As seeds, they only contain one seed leaf. (Seed leaves, or cotyledons, are the first leaves to grow from a seed.)
Examples of monocots include grasses, lilies, tulips and palms. Around 23% of all angiosperms are monocots.
Dicots, also known as dicotyledons, have two seed leaves. Examples of dicotyledons include: beeches, oaks, maples, roses, dandelions, daisies, cabbages, apples, cherries and magnolias (and most other trees and shrubs that aren’t conifers).
- You can find out more about plant reproduction on this page: The Life Cycle Of A Plant
Plants reproduce either asexually or sexually.
In asexual reproduction, the plant produces offspring that is genetically identical to itself.
Asexual reproduction is faster and requires less energy than sexual reproduction because the plant involved is not required to find another plant with which to reproduce.
In sexual reproduction, a male sperm cell fuses with a female egg cell, producing an embryo that contains characteristics of both parents.
Sexual reproduction results in a varied population that is more resilient to threats such as disease or environmental change.
The way in which plants reproduce – both sexually or asexually – differs depending on the type of plant.
Asexual reproduction involves the plant making an exact copy, or clone, of itself. It is a very common means of reproduction in plants.
Plants have evolved various organs to allow themselves to reproduce asexually. Most plants produce either modified stems or modified roots from which new individuals can grow.
An example of a modified stem is a rhizome, which grows underground and produces buds from which new plants grow. Ferns and many types of grasses produce rhizomes.
A tuber, such as a potato, is another example of a modified stem from which offspring can grow.
Sexual reproduction in plants is slightly more complicated than that in animals, as it involves the creation of two generations per life cycle in a system known as the alternation of generations.
On this page we cover the basics of plant reproduction. Future articles will cover the subject in much greater depth.
Sexual Reproduction In Seedless Plants
Bryophytes and seedless vascular plants such as ferns reproduce sexually by dispersing spores.
The spores grow into new plants which contain both male and female parts. Sperm (male reproductive material) from the male part of one plant fuses with the egg in the female part of another plant. This results in an embryo that contains genetic material from both parents.
These types of plant are often confined to damp environments, as the sperm have to swim through water in order to reach the egg of another plant.
Sexual Reproduction In Seed-Producing Plants
Gymnosperms and flowering plants grow from seeds. In gymnosperms such as conifers, the seeds are housed in cones, and are released when the cone ripens. The seeds of flowering plants are protected and dispersed within a fruit.
Seeds are produced as a result of pollination.
Pollen is a powdery substance that contains and protects a plant’s male reproductive material. In gymnosperms it is produced in male cones.
In flowering plants pollen is produced by the anther, part of the male reproductive structure of a flower.
In gymnosperms, pollen is usually carried by the wind, with pollination occurring when the pollen reaches an ovule in a female cone.
In flowering plants, pollination occurs when pollen adheres to the stigma, which is part of the female structure in a flower. Here it will germinate and transport sperm to the egg.
Many flowering plants rely on animals such as insects or birds to transport pollen between the flowers of different plants of the same species, thereby enabling reproduction to take place. Animals that fulfil this role are known as pollinators.
Flowers attract pollinators by producing nectar, a sugary, energy-providing substance, and excess pollen, which itself is an important source of food for many animals.
While the pollinator is collecting a flower’s nectar, its body also picks up grains of pollen as it brushes against the flower’s anthers.
When the pollinator visits another flower, the pollen it carries is deposited on the new flower’s stigma. Not only has the pollinator had a free meal, but it has also helped the plants to reproduce!
Cross-Pollination & Self-Pollination
Cross-pollination occurs when pollen is transported to a different plant. Self-pollination, in which pollen is transported to flowers of the same plant (or the female parts of the same flower), can also occur.
Plants: The Ultimate Guide – Conclusion
We hope that by reading this article your knowledge and appreciation of plants has grown. Plants are fascinating organisms which perform a vital role in virtually all terrestrial ecosystems.
You can find out more about the amazing world of plants on the following pages: