Plant: Definition, Evolution, Taxonomy

Plants are some of the oldest forms of life on Earth. Whether they're indoor plants, plants in your home garden, native plants to your area or tropical plants, they use the pigment chlorophyll to capture the sun’s energy to make food.

Of the six kingdoms classifying all organisms in taxonomy, plants are, as you'd guess, in the Kingdom Plantae. Plants are one of the main producers of oxygen in the atmosphere.

Definition of Plants

Plants are multicellular, eukaryotic organisms that grow from embryos. Plants use the green pigment chlorophyll to capture sunlight. In turn, plants use the sun’s energy to make sugars, starches and other carbohydrates as food.

They also use this energy for other metabolic purposes. Plants are considered photoautotrophic, since they can make their own food.

One distinguishing feature of plants is that they cannot move like animals and bacteria. Because of their inability to move out of their current location, plants cannot relocate in difficult circumstances.

This is why plant care is difficult and depends on people to get the amount of light (full sun, medium light, etc), water levels and other environmental conditions right for the plants to thrive. Their sedentary nature makes it necessary for plants to develop adaptations to cope with their surrounding environments.

Plants possess a rigid boundary to their cells, called a cell wall. Inside the cell there is a large central vacuole and plasmodesmata. The plasmodesmata are little holes through which water and nutrients can center the cell through diffusion.

Other plant cell features include a nucleus, mitochondria and other organelles. The cell wall is made of cellulose, which is both relatively rigid yet has some flexibility.

Plants exist throughout the world, except for the deep parts of the ocean, extremely arid deserts and parts of the Arctic.

Plants of the world include seedless non-vascular plants, seedless vascular plants and plants with seeds.

Taxonomy/Classification of Plants

Plants are living things and are members of the Kingdom Plantae. They are classified based on whether they circulate fluids into non-vascular or vascular plants.

Vascular plants contain a circulatory system, using a structure called xylem to carry nutrients and water throughout the plant. In non-vascular plants, this type of structure does not exist. This is why non-vascular plants require easily accessible sources of moisture in order to survive.

Plants reproduce differently from other organisms as well, using alternation of generations. Diploid plants or sporophytes start their development in the haploid plant or gametophyte phase. The size of these different forms is one of the characteristics that helps distinguish non-vascular and vascular plants.

Non-Vascular Plants

Non-vascular plants or bryophytes include mosses, liverworts and hornworts. Non-vascular plants do not have flowers or seeds; instead, they reproduce via spores. In the bryophytes, the sporophyte part of the plant is small, and the gametophyte is the dominant part of the plant.

Non-vascular plants tend to be low-growing and do not possess true root systems. Non-vascular plants grow along the ground, covering rocks and other substrate.

Land plants have developed different adaptations for the prevalence or lack of water in their surroundings. In the case of non-vascular plants, the tendency to dry out can be protective. This is called desiccation tolerance. Mosses and liverworts can recover from drying out in a short period of time.

Vascular Plants

In contrast to non-vascular plants, vascular plants contain xylem and phloem, structures used to transport liquids and nutrients throughout the body of a plant. Vascular plants are also referred to as tracheophytes.

Vascular plants also produce seeds and flowers, although some of them do produce spores as well. The pteridophytes have sporophytes that go on to be independent plants.

Spermatophytes are the seed plants. They make up the majority of plants. These are characterized by having small gametophyte forms.

Vascular plants have their own methods for storing water and dealing with water loss. Succulent plants, for example, have tissues that swell and store water in arid environments. Examples of succulents include cacti and agave plants.

Vascular plants also have adapted chemicals and structures like spines to deter other organisms from eating them.

Vascular plants can further be categorized according to seed prevalence. Seedless vascular plants include ferns and horsetails. Seedless plants prefer moist locations and reproduce via spores, similar to non-vascular plants.

Vascular plants with seeds are subdivided into conifers (gymnosperms) and flowering or fruit-bearing plants. Conifers possess naked seeds in cones and do not produce fruit or flowers. Conifers include pines, firs, cedars and ginkgo.

Seed plants that have flowers or fruit covering their seeds are called angiosperms. Today, angiosperms dominate the plant world.

Examples of vascular plants include grasses, trees, ferns and any plants with flowers.

Evolution of Plants on Earth

Plants evolved over time to include more advanced physical characteristics, reproduction methods, seeds and flowers. Those who study the evolution of plants are called paleobotanists.

Green algae spurred the evolution of plants. Green algae organisms do not have waxy cuticles or cell walls like more advanced plants.

Charophytes, known by their common name of green algae, also differed from more advanced plants by having different mechanisms for cell division. They also lived chiefly in water. Diffusion served the algae well for nutrient delivery. (Those algae that are single-celled are not considered plants.)

Moving from Water to Land

It is thought that the movement from water to land necessitated ways to cope with desiccation. This meant being able to disperse spores into the air, finding ways to stay upright and attached to substrates, and creating methods to capture sunlight to make food. Having access to more sunlight by being on land proved to be advantageous.

Another issue plants had to contend with was a lack of buoyancy once outside of water. This necessitated stems and other structures to lift the plant. Protective adaptations to contend with ultraviolet radiation also had to be developed.

Alteration of Generations

The chief adaptations of land plants, or embryophytes, include the alteration of generations, the sporangium (for spore formation), the antheridium (haploid cell producer) and apical meristem for shoots and roots. The alteration of generations entails the plants having both haploid and diploid stages in their life cycle.

Seedless plants use the male antheridium to release sperm. Those swim to the female archegonia to fertilize the egg. In seed plants, pollen take on the role of reproduction.

Non-vascular plants have diminished sporophyte stages. In vascular plants, however, the gametophyte stage is prevalent.

Adaptations for Plants to Land

Other adaptations arose as well. For example, seed plants do not need as much water as the more primitive seedless plants. The apical meristem contains a tip that hosts rapidly dividing cells to increase its length. This means the shoots can better reach more sunlight, and the roots can better access nutrients and water in the ground.

Another adaptation, the waxy cuticle on plant leaves, helped prevent loss of water. Stomata, or pores, developed to allow gases and water to enter and exit the plant.

Eras of Plant Evolution

The Paleozoic Era heralded the rise of plants. This era is delineated into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods of geologic time.

Land plants have existed since the Ordovician Period, nearly 500 million years ago. The fossil record reveals the cuticles, spores and cells of those first land plants. Modern plants arrived around the Late Silurian Period.

Liverworts are thought to have been the earliest example of land plants. This is partly due to the fact they are the only land plant with no stomata.

Plants evolved embryo protection prior to vascular structure. The major shift of plants to become vascular was soon followed by the development of seeds and flowers.

The Devonian Period (roughly 410 million years ago) heralded the vast array of vascular plants that more resembles the modern landscape. Many early bryophytes subsisted on wet mudflats.

Changing Plant Relationships and Structures

Being on land gave plants better access to carbon dioxide. The increased vegetation of the Devonian led to greater atmospheric oxygen. This helped the eventual rise of animals on the landscape, which needed oxygen to breathe.

During this time, some plants entered symbiotic relationships with fungi. This aided the plants’ roots.

During the Silurian Period, a shift to stems and branches had occurred in plants. This allowed plants to grow taller to reach more light. In turn, taller stems required stiffer structures until trunks eventually developed.

An early vascular plant from his period was Cooksonia. This plant did not have leaves, but it did bear spore sacs on the ends of stems.

This period has yielded significant evidence of developments from its fossil record. Some other early vascular plants included Zosterophyllophyta (clubmoss predecessors) and Rhyniophyta (predecessors of Trimerophytophyta and other leafy plants).

They likely did not have true roots and leaves, and were more similar to mosses. While most of these were low-growing plants, trimerophytes sometimes grew as high as a meter.

The Carboniferous Period

Ferns, horsetails, seed plants and trees began to take precedence during the Carboniferous Period, about 300 million years ago. Horsetails (Calamites) even reached several meters in height.

Deltas and tropical swamps of the Carboniferous Period played host to new plants and forests. These swamp forests decayed and eventually formed into the swaths of coal deposits around the world.

The earliest seed plants, or gymnosperms, developed during the Carboniferous as well. Conifers, tree ferns (Psaronius) and seed ferns (Neuropteris) grew in the coal forests of this era. Large insects and amphibians thrived among these new forests.

Once animals arrived on land, plants had predators. Further adaptations by plants developed for self-protection. Plants developed complex organic molecules that made them taste bad to animals; some even made the plants toxic. In contrast, other plants co-evolved with animals that helped them pollinate or disperse their fruits and seeds.

The First Flowering Plants

The early Cretaceous Period (around 130 million years ago) saw the rise of conifers, cycads and similar plants, tree ferns and small ferns. The Cretaceous and Jurassic periods witnessed the domination of such gymnosperms. The first angiosperms, or flowering plants, arose during the Cretaceous. One example is that of Silvianthemum suecicum (an ancient type of saxifrage).

Once flowering plants took hold in the prehistoric landscape, they quickly became the most successful plants. They diversified rapidly from the tropical areas and spread around the world by the Paleogene, a period of time that encompasses the early Tertiary Period (about 50 million years ago). Today, 250,000 of the 300,000 species of plants are angiosperms.

During the Palaeogene, many new species arose, such as mangroves, magnolia and Hibbertia. By this time, the number of birds and mammals had grown substantially. At this point, the plants of the world greatly resembled those of the modern era.

The gnetophytes were the last major gymnosperms to arrive. During the Neogene, or the latter portion of the Tertiary Period, grass appeared. Eventually forested regions changed along with the climate, and areas of savanna began to appear.

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