Plants

Plants are thought to have evolved from green Algae around 470 million years ago (mya) during the Ordovician period. The closest living relatives to land plants today are known as charophytes, a type of green Algae.

The first plants are thought to have been similar to current liverworts and mosses. This means that they were non-vascular and had shallow roots.

If a plant is classified as non-vascular, they don't have specialized structures for transporting nutrients and water throughout their bodies. These plants fall under the category of Bryophytes. Bryophytes are considered the earliest land plants.

After bryophytes, Pteridophytes evolved. Pteridophytes include ferns, horsetails, and lycophytes. They are known for being Vascular Plants that disperse spores.

Once pteridophytes evolved, Gymnosperms came next. Gymnosperms are the first land plants with seeds, including conifers, Ginkgo, and cycads.

Angiosperms, the most common type of green plant, came after gymnosperms. They also have seeded plants, except they produce flowers and fruits and contain endosperm. The endosperm is a tissue that encompasses and gives nutrients to the embryo within its seeds. Since gymnosperms have no flowers or fruits, they have naked seeds.

Aquatic plants include plants like the water lily and the lotus. They are plants that are either fully submerged or floating in the water. Aquatic plants are thought to have emerged from terrestrial plants. Algae are not considered aquatic plants because they don't have true leaves, roots, and stems. We will go over the plants' structures in detail in the section parts of plants.

Characteristics of Plants

The following are the defining characteristics of land and aquatic plants.

Characteristics of land plants:

• They are autotrophic organisms that make their food from chemical substances. They do this through the process of Photosynthesis, which uses light energy to convert carbon dioxide \((CO_2)\) and water \((H_2O)\) into Carbohydrates or sugars. Plants specifically store their food as starch, a complex carbohydrate.

• Land plants contain chlorophyll, a green pigment that allows plants to capture sunlight and convert it into chemical energy.

• Plants have cell walls that are made of cellulose. Cellulose is the most common complex carbohydrate made of glucose molecules bound together. Glucose is a simple carbohydrate or building block of Carbohydrates.

• These plants are not motile or cannot move. They are oriented in one place by their roots but can respond to the sun or touch. When plants move in the sun's direction, this is called heliotropism.

• Tropism occurs when living things grow or move in response to environmental stimuli. An example of tropism is heliotropism.

• A plant's life cycle can be described as an alternation of generations. We will go over this more in-depth under the section Reproduction of plants.

• Most of the plants are terrestrial; they make up around 80% of the planet's Biomass. Land plants include bryophytes, pteridophytes, gymnosperms, and Angiosperms. Angiosperms are the most diverse and common type of terrestrial plants.

• All land plants have adaptations to help them survive on land. They have waxy cuticles, root-like structures or roots, Stomata, association with Fungi, and life cycles that alternate. We will dive into this topic more deeply in the following few sections.

Characteristics of aquatic plants:

• Aquatic plants are thought to have adapted from terrestrial plants due to the challenges presented on land.

• They undergo Photosynthesis, but due to the restriction of carbon dioxide in the water, some have evolved to break down bicarbonate ions instead.

• Their general morphology consists of aerenchyma Cells, allowing them to maintain buoyancy. Aerenchyma cells are formed from a modification of parenchyma cells to form a spongy tissue with air holes in the stems, leaves, and roots, which allow gas exchanges to occur.

• Aquatic plants are less rigid, have thin or no cuticles and have smaller roots because of being constantly in or near water.

• These plants can be emergent, fully submerged, floating with root systems attached to the bottom of the water, and free-floating.

Parts of Plants

Since most plants are terrestrial, for parts of plants, we will focus on land plants.

Most land plants share these essential parts: stems, flowers, roots, leaves, fruits, and seeds, as shown in Figure 1.

• Stems act as structural support for the rest of the plants and transport nutrients and water. In some plants, such as bananas, stems are where photosynthesis occurs.

• Flowers are the parts of angiosperms that are seed-bearing and contain reproductive organs (stamen and carpels) surrounded by bright petals that attract insects to pollinate them.

• Roots are the part of the plants attached to the ground and absorb nutrients and water for the rest of the plant.

• Leaves are usually where the process of photosynthesis of most plants takes place. Chlorophyll, the pigment that makes plants green, captures sunlight and converts it into chemical energy (food) and oxygen.

• Fruits are sweet and fleshy products that surround the seed of some angiosperms. They protect the seeds and allow Animals to eat them, encouraging seed dispersal.

• Seeds are gymnosperms' and angiosperms' way of reproducing. They contain the embryo required to produce another plant. Seeds have a protective coat and cotyledons, which are the seed leaves within the embryos of a seed that provide the embryo with nutrition to grow.

Figure 1: Parts of plants illustrated. Image by brgfx on Freepik.

Examples of Plants

There are many plants on this planet, and, as previously mentioned, they are primarily terrestrial. In this section, we'll go over some commonly seen plants.

• Mosses

  • Mosses are bryophytes or non-vascular and flowerless plants. They often form into mats or clumps in damp areas since they don't have specialized structures for transporting nutrients and water throughout their bodies, as shown in Figure 2.

  • They are non-woody, tiny plants that absorb nutrients and water, usually through their leaves, which perform photosynthesis.

  • Since they have no seeds or flowers, mosses reproduce through spores. Their stems are either branched or simple.

  • Like other bryophytes, they are primarily haploids, having only a single set of chromosomes.

Figure 2: Moss-on-rock illustration. PublicDomainPictures.net, Kevin Casper.

• Pine trees

  • Since there are many trees, we'll focus on pine trees which are gymnosperms.

  • Pine trees are evergreen, coniferous trees with resin that live in sandy and dry soils.

  • The branches are arranged in whorls or ordered in circles around the trunk; a new whorl is added each year.

  • Pines, on average, live around 400 years, and white pines are the most common type since people often use them as Christmas trees.

  • White pines typically have short and shiny needles, occur in groups of five, and have long and slender cones.

  • Resin is a viscous substance excreted by plants that protect them from insects and other pathogens. Resin can be used as glue or adhesives in construction and buildings commercially.

• Roses

  • Instead of focusing on all flowers, we'll narrow the scope to one of the most well-known flowers: roses. Roses are angiosperms or flowering plants.

  • Roses are plants that belong to the family Rosaceae and are native to temperate regions of the Northern Hemisphere.

  • People commonly associate roses with thorns, but they do not have thorns; instead, they have prickles to help protect them against predators attracted to their sweetly scented fragrance.

  • Roses come in various colors, with red roses being the most common due to their symbol of romance.

Importance of Plants

Through photosynthesis, plants provide us with oxygen. It's estimated that one large tree can give around four people oxygen to breathe and survive. Plants also absorb carbon dioxide, a greenhouse gas that has increased global warming due to humans burning fossil fuels for gas and energy.

Plants also provide other Animals and us with food, shelter, medicine, fuel, etc.

Plants are primary producers that we need to consume to obtain energy. Examples of plants we eat for energy are potatoes, bananas, and other vegetables and fruits. Meats we eat, such as beef, must also consume plants like grass. Without plants, we would not have many different animals to consume.

Plants and animals also have a mutualistic, or interdependent, relationship from which both benefit. For instance, bees are pollinators that flowering plants rely on to reproduce.

Plant matter has the potential to work as biofuels or renewable sources of energy that could replace fossil fuels. This means that plants could potentially power our future without all the contamination from burning fossil fuels, which cannot be renewed.

The ironic part is that fossil fuels were made from decomposing plants and animals over millions of years underground.

Reproduction of Plants

We'll focus on the general life cycle of plants since these details apply to all plants:

Plants' reproductive cycle can be described as an alternation of generations.

Gametes are mature haploids that are either male or female sex Cells that fuse to form a zygote. It's equivalent to the egg Cells women have and sperm men have that need to combine to produce a zygote.

Spores are unicellular reproductive cells that can grow into a new individual without sexual Reproduction or fusion.

The gametophyte or multicellular haploid stage makes gametes via a process called Mitosis. They combine to form a diploid zygote. The zygote then grows into a multicellular and diploid sporophyte that produces haploid spores through a Meiosis process. The haploid spores then develop into gametophytes again, as shown in Figure 3.

The alternation of generations does differ in different types of plants:

• Bryophytes have a life cycle dominated by the gametophyte because the sporophyte is attached to and dependent on the gametophyte.

• In pteridophytes, sporophytes are more significant than the gametophyte, but both are free-living, unlike bryophytes.

• Gymnosperms and angiosperms both have a life cycle dominated by sporophytes. The difference is that angiosperms have double fertilization. Double fertilization is when one sperm cell fertilizes the egg cell while the other sperm cell combines with two polar nuclei to form an endosperm.

Figure 3: Alternation of generations life cycle illustrated. Wikimedia, Peter coxhead.