Figure 1. A locust leaf consists of leaflets arrayed along a central midrib. Each leaflet is a complex photosynthetic machine, exquisitely adapted to capture sunlight and carbon dioxide. An intricate vascular system supplies the leaf with water and minerals, and exports the products of photosynthesis. (credit: modification of work by Todd Petit)
Plants are as essential to human existence as land, water, and air. Without plants, our day-to-day lives would be impossible because without oxygen from photosynthesis, aerobic life cannot be sustained. From providing food and shelter to serving as a source of medicines, oils, perfumes, and industrial products, plants provide humans with numerous valuable resources.
When you think of plants, most of the organisms that come to mind are vascular plants. These plants have tissues that conduct food and water, and they have seeds. Seed plants are divided into gymnosperms and angiosperms. Gymnosperms include the needle-leaved conifers—spruce, fir, and pine—as well as less familiar plants, such as ginkgos and cycads. Their seeds are not enclosed by a fleshy fruit. Angiosperms, also called flowering plants, constitute the majority of seed plants. They include broadleaved trees (such as maple, oak, and elm), vegetables (such as potatoes, lettuce, and carrots), grasses, and plants known for the beauty of their flowers (roses, irises, and daffodils, for example).
While individual plant species are unique, all share a common structure: a plant body consisting of stems, roots, and leaves. They all transport water, minerals, and sugars produced through photosynthesis through the plant body in a similar manner. All plant species also respond to environmental factors, such as light, gravity, competition, temperature, and predation...
Plant Cells
Why do plant cells look like little rectangles? Look at Figure 1 and notice how all the cells seem to stack on each other, with no spaces in between. Might this allow the cells to form structures that can grow upright?
Figure 1. A section of a pine embryo.
Organs in Plants?
Your body includes organ systems, such as the digestive system, made of individual organs, such as the stomach, liver, and pancreas, which work together to carry out a certain function (in this case, breaking down and absorbing food). These organs, in turn, are made of different kinds of tissues, which are groups of cells which work together to perform a specific job. For example, your stomach is made of muscle tissue to facilitate movement and glandular tissue to secrete enzymes for chemical breakdown of food molecules. These tissues, in turn, are made of cells specialized in shape, size, and component organelles, such as mitochondria for energy and microtubules for movement.
Plants, too, are made of organs, which in turn are made of tissues. Plant tissues, like ours, are constructed of specialized cells, which in turn contain specific organelles. It is these cells, tissues, and organs that carry out the dramatic lives of plants.
Plant Cells
Plant cells resemble other eukaryotic cells in many ways. For example, they are enclosed by aplasma membrane and have a nucleus and other membrane-bound organelles. A typical plant cell is represented by the diagram in Figure 2.
Figure 2. Plant cells have all the same structures as animal cells, plus some additional structures. Can you identify the unique plant structures in the diagram?
Plant Cell Structures
Structures found in plant cells but not animal cells include a large central vacuole, cell wall, and plastids such as chloroplasts.
- The large central vacuole is surrounded by its own membrane and contains water and dissolved substances. Its primary role is to maintain pressure against the inside of the cell wall, giving the cell shape and helping to support the plant.
- The cell wall is located outside the cell membrane. It consists mainly of cellulose and may also contain lignin, which makes it more rigid. The cell wall shapes, supports, and protects the cell. It prevents the cell from absorbing too much water and bursting. It also keeps large, damaging molecules out of the cell.
- Plastids are membrane-bound organelles with their own DNA. Examples are chloroplasts and chromoplasts. Chloroplasts contain the green pigment chlorophyll and carry out photosynthesis . Chromoplasts make and store other pigments. They give flower petals their bright colors.
Types of Plant Cells
There are three basic types of cells in most plants. These cells make up ground tissue, which will be discussed in another concept. The three types of cells are described in table below. The different types of plant cells have different structures and functions.
| Type of Cell | Structure | Functions | Example |
|---|---|---|---|
| Parenchymal |
cube-shaped
loosely packed
thin-walled
relatively unspecialized
contain chloroplasts
|
photosynthesis
cellular respiration
storage
|
food storage tissues of potatoes
 |
| Collenchymal |
elongated
irregularly thickened walls
|
support
wind resistance
|
strings running through a stalk of celery
 |
| Sclerenchymal | very thick cell walls containing lignin |
support
strength
|
tough fibers in jute (used to make rope)
 |
Plant Tissues
Figure 1. What is this abstract pattern? Is it just a random artistic piece? Is it a depiction of a pattern of bubbles? Would you believe it is part of a plant? It is actually the center portion of a carrot taproot. And these are all cells. Cells that have come together to form a tissue, with a specific function. What do you think is the main function of tissue in a plant’s root?
Plant Tissues
As for all animals, your body is made of four types of tissue: epidermal, muscle, nerve, and connective tissues. Plants, too, are built of tissues, but not surprisingly, their very different lifestyles derive from different kinds of tissues. All three types of plant cells are found in most plant tissues. Three major types of plant tissues are dermal, ground, and vascular tissues.
Dermal Tissue
Dermal tissue covers the outside of a plant in a single layer of cells called the epidermis . You can think of the epidermis as the plant’s skin. It mediates most of the interactions between a plant and its environment. Epidermal cells secrete a waxy substance called cuticle , which coats, waterproofs, and protects the above-ground parts of plants. Cuticle helps prevent water loss, abrasions, infections, and damage from toxins.
This tissue includes several types of specialized cells. Pavement cells, large, irregularly shaped parenchymal cells which lack chloroplasts, make up the majority of the epidermis. Within the epidermis, thousands of pairs of bean-shaped schlerenchymal guard cells swell and shrink by osmosis to open and close stomata, tiny pores which control the exchange of oxygen and carbon dioxide gases and the release of water vapor. The lower surfaces of some leaves contain as many as 100,000 stomata per square centimeter.
Figure 2. The epidermis of Arabidopsis shows both pavement cells (A) and stomata made of sclerenchymal guard cells (B), which control water loss and gas exchange.
Ground Tissue
Ground tissue makes up much of the interior of a plant and carries out basic metabolic functions. Ground tissue in stems provides support and may store food or water. Ground tissues in roots may also store food.
Vascular Tissue
Figure 3. Bundles of xylem and phloem run through the ground tissue inside this stalk of celery. What function do these tissues serve?
Vascular tissue runs through the ground tissue inside a plant. Your body was able to grow from a single cell to perhaps 100 trillion cells because, 21 days after fertilization, a tiny heart began to pump blood throughout your tiny self – and it hasn’t stopped since. The blood it pumps carries water, oxygen and nutrients to each one of your trillions of cells, and removes CO 2and other wastes. Of course plants don’t have hearts, but they do have vessels that transport water, minerals, and nutrients through the plant. These vessels are the vascular tissue, and consist of xylem and phloem. Xylem and phloem are packaged together in bundles, as shown in figure 3.
Summary
- The three types of plant cells are found in each of the major types of plant tissues: dermal, ground, and vascular tissues.
- Dermal tissue covers the outside of a plant in a single layer of cells called the epidermis. It mediates most of the interactions between a plant and its environment.
- Ground tissue makes up most of the interior of a plant. It carries out basic metabolic functions and stores food and water.
- Vascular tissue runs through the ground tissue inside a plant. It consists of bundles of xylem and phloem, which transport fluids throughout the plant.
