Lesson Explainer: Kingdom Plantae Biology

In this explainer, we will learn how to recall the characteristics of organisms belonging to kingdom Plantae.

Earth has an incredibly diverse range of living organisms. To make these organisms easier to study, scientists have classified them into distinct groups according to their visible characteristics or evolutionary history. This process is called taxonomy. All living organisms that have been discovered so far can be classified into kingdoms, which are groups containing a large number of organisms sharing some characteristics.

Definition: Taxonomy

Taxonomy is the scientific study of biological classification.

In 1969, a scientist called Robert Whittaker proposed the five-kingdom classification system, outlined in Figure 1.

Kingdom Plantae is a large taxonomic group that contains all the species that we commonly refer to as plants. Scientists estimate that we have so far identified nearly 400‎ ‎000 different species of plants. Plants inhabit almost every environment on Earth. Without plants producing oxygen for us to breathe, human life would cease to exist.

All species of plants have some characteristics in common. Plants are eukaryotes, which means their cells contain a membrane-bound nucleus and other membrane-bound organelles. Their cells are surrounded by a cell wall, which provides structure and support. The main component of these walls is the polymer cellulose. Plants are autotrophic; they make their own food via a process called photosynthesis. To be able to carry out this process, one of the membrane-bound organelles that they contain in their cells is chloroplasts.

Definition: Autotroph

An autotroph is an organism that can synthesize its own food from inorganic materials.

Definition: Photosynthesis

Photosynthesis is the process by which carbon dioxide and water are converted into sugars, such as glucose, and oxygen in the presence of sunlight.

Example 1: Recalling the Key Characteristics of Kingdom Plantae

The picture provided shows an organism belonging to kingdom Plantae. What are the characteristics of organisms belonging to this kingdom?

Aloe vera plants
  1. These organisms are eukaryotic, multicellular, and heterotrophic.
  2. These organisms are prokaryotic, multicellular, and autotrophic.
  3. These organisms are prokaryotic, multicellular, and heterotrophic.
  4. These organisms are eukaryotic, multicellular, and autotrophic.

Answer

Kingdom Plantae includes all the organisms that we commonly refer to as plants. You are most likely surrounded by plants on a daily basis; without plants continually producing oxygen, there is no way humans can survive on Earth. The process that plants use to produce oxygen is photosynthesis, and this is also the process that they use to obtain their own nutrition. This makes plants autotrophic, as they can make their own food. Plants are eukaryotes, which means that they contain membrane-bound organelles, importantly a membrane-bound nucleus. Looking at the image of the plant above, we can see that it is relatively large and has multiple different extensions, giving it a rather complex shape. We can, hence, conclude that these organisms are highly likely to be made up of more than one cell (multicellular).

Therefore, we can conclude that organisms belonging to kingdom Plantae are eukaryotic, multicellular, and autotrophic.

In this explainer, we are going to look at groups of plants in more detail, including higher algae and nonvascular and vascular plants. Some more recent classification systems now place higher algae in kingdom Protista, but we are going to have a look at their characteristics here.

Higher algae are photosynthetic organisms that inhabit aquatic environments. Phyla that are considered higher algae include Rhodophyta, Phaeophyta, and Chlorophyta. Table 1 summarizes their key characteristics and provides some example organisms.

The photo below shows Spirogyra as viewed under a light microscope. You can see from the image that this Spirogyra has spiral-shaped chloroplasts in its cells.

Spirogyra

Figure2

Plants that are not considered higher algae include the phyla Bryophyta and Tracheophyta. Figure 3 shows a very basic outline of their evolutionary relationships.

The organisms belonging to phylum Bryophyta are nonvascular plants. This means that they do not have complex or specialized vascular tissues, which are cells joined together into a network of tubes used to transport water, minerals, and nutrients around the body of the plant. Bryophytes may also have structures called rhizoids, which extend from their lower epidermal cells to help anchor the plant into the ground. Examples of nonvascular plants include the moss Funaria hygrometrica, shown in the photo below, which live in very moist environments to ensure that they have constant access to water and minerals.

Funaria hygrometrica

Figure4

Organisms belonging to phylum Tracheophyta, however, do have specialized vascular tissues. These include the phloem and the xylem, which are used to transport sugars and water respectively.

Example 2: Using Our Knowledge of Bryophyta and Tracheophyta to Classify a Given Organism

A flower with a long stem that contains a specialized system for transporting water and sugars has been found. What phylum, Bryophyta or Tracheophyta, is this plant most likely to belong to?

Answer

Any plant that has a specialized system for transporting water and sugars is said to have a vascular system. This is common in flowering plants with long stems and trees with long trunks, rather than in small plants close to the ground, such as mosses. We know that plants belonging to phylum Tracheophyta are called vascular plants and have specialized transport tissues.

Therefore, the plant being described must belong to phylum Tracheophyta.

Phylum Tracheophyta can be further divided into three classes: Filicatae, Gymnospermae, and Angiospermae.

Plants belonging to class Filicatae in phylum Tracheophyta are more commonly known as ferns. The majority of ferns are found as herbs, and some are shrubs or trees. They have stems, leaves, and roots extending through the ground. Ferns do not produce flowers or seeds but instead reproduce using small reproductive structures called spores. Their leaves have a pinnate shape, which you can see in the photo of a commonly found fern below. Ferns also include the plant genera Polypodium and Adiantum.

fern petals

Figure5

Plants belonging to class Gymnospermae are largely trees. These plants produce seeds that are not enclosed in other structures, which is in contrast to many other classes of plants. In fact, the word gymnosperm comes from the Greek words for “naked” and “seeds,” so the name literally translates to “naked seeds.” Similar to ferns, gymnosperms do not produce flowers, but they do carry cones, which can be male or female. They have simple leaves that form needle shapes. Examples of gymnosperms are the firs and pines that people often use as Christmas trees. The photo below shows a close-up of the branches of a Pinus flexilis tree.

Limber Pine

Figure6

Angiospermae is a class of Tracheophyta that includes flowering plants. These plants inhabit the land and produce seeds, leaves, and fruits. This class can be further divided into two groups: monocotyledons and dicotyledons.

A cotyledon is referred to as the embryonic leaf of a plant. It is an important part of the embryo within a plant seed. Scientists can use this part to classify angiosperms into monocotyledons (seeds with one cotyledon) and dicotyledons (seeds with two cotyledons).

Definition: Cotyledon

A cotyledon is the embryonic leaf of a plant contained within the plant seed.

Example 3: Applying the Taxonomic Hierarchy to Classify a Given Organism

The picture provided shows a bamboo plant, which is a monocotyledon. Which of the following is the correct order of taxonomic classification, from largest to smallest, for the bamboo plant?

Bamboo plant
  1. Plantae, Tracheophyta, Angiospermae, monocotyledons
  2. Tracheophyta, Plantae, Angiospermae, monocotyledons
  3. Monocotyledons, Plantae, Tracheophyta, Angiospermae
  4. Plantae, Angiospermae, Tracheophyta, monocotyledons

Answer

The order of the taxonomic hierarchy moves from the largest group, in that it contains the largest number and widest variety of species, to the smallest, most specific group. In this case, the largest group is the kingdom, which is kingdom Plantae. This means that we can immediately eliminate any answer options that do not start with Plantae. Kingdom Plantae is broadly divided into higher algae, tracheophytes (vascular plants), and bryophytes (nonvascular plants). Bamboo is a vascular plant, as it has specialized vascular tissues for transport. Tracheophytes can be further divided into classes Filicatae, Gymnospermae, and Angiospermae. Bamboo has leaves and seeds and can produce flowers, so it is classified as an angiosperm. Angiosperms can be divided into two groups depending on how many embryonic leaves, or cotyledons, they have in their seeds. Bamboo has one, so it is classified as a monocotyledon.

Therefore, the correct order of taxonomic classification for a bamboo plant is Plantae, Tracheophyta, Angiospermae, monocotyledons.

We now know that monocotyledons and dicotyledons differ in their seed structure. They also have other important structural and functional differences, which are outlined in Figure 7.

As we can see, both monocots and dicots produce seeds, flowers, and leaves and have specialized vascular systems. However, there are some key differences: these vascular bundles are arranged in rings in the stem of a dicot but are scattered in the stem of a monocot. The leaves of a monocot are narrow with parallel veins, but a dicot has pinnate or palmate-shaped leaves. The roots of a monocot are branching and fibrous, but a dicot is likely to have one main, or tap, root that smaller roots branch from.

Example 4: Identifying Monocotyledons and Dicotyledons from Images

The diagram shows some key features of a plant. Is this plant likely to be a dicot or a monocot?

Answer

Monocots (or monocotyledons) and dicots (or dicotyledons) are two groups of plants that belong to class Angiospermae. Angiosperms are flowering plants that have leaves and vascular systems and produce seeds. Plants can be classified as a monocot or a dicot depending on various characteristics.

A monocot has floral organs that generally occur in multiples of three. Their leaves are narrow with veins running parallel to each other, and their roots are fibrous and branching. Dicots have floral organs that occur in multiples of four or five. Their leaves are likely to be pinnate or palmate (so they have a much broader shape with leaflets extending out from a central point). They also have a larger main root, or a tap root. If we look at the diagrams provided, we can see that the leaf is narrow, with parallel veins, and that the roots are highly branched.

Therefore, the plant in question is likely to be a monocot.

Let’s summarize what we have learned about the organisms that belong to kingdom Plantae.

Key Points

  • Organisms belonging to kingdom Plantae are heterotrophic, eukaryotic, and multicellular.
  • Kingdom Plantae can be divided into the phyla of higher algae (Rhodophyta, Phaeophyta, and Chlorophyta) and phyla Tracheophyta and Bryophyta.
  • Phylum Tracheophyta can be divided into classes Filicatae, Gymnospermae, and Angiospermae.
  • Class Angiospermae contains flowering plants and can be divided into monocotyledons and dicotyledons.

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