Plant Structure

 Plants exhibit great variety. Cacti in the deserts, pines in the mountains, water lilies in a pond, and wheat in a field. Yet among all these different types of plants there are basic structures shared by nearly all of them. Almost all plants have a root system and a shoot system. Root systems are composed of roots (obviously) and shoot systems are composed of leaves and stems. Regardless of the overall morphology (form and structure) of a plant they all rely on these structures to survive and thrive.

Roots serve a few purposes. They anchor the plant in the ground. Roots allow redwoods to grow hundreds of feet into the air. They provide the support that keeps riparian vegetation from washing out during a flood. Roots also provide the entire plant with water. Roots pull the water out of the ground and transport it to the rest of the plant through its vasculature system. Roots are also responsible for the uptake of nutrients that the plant needs in large and small quantities.

Stems represent the (generally) aboveground structures that leaves sprout from. Stem structure is composed of a series of nodes and internodes. Nodes are the point where leaves sprout. The internodes are the space between. Stems have two types of buds present. (A bud is the point where new stem growth occurs.) A terminal bud represents the growth point of a stem. Usually a terminal bud is located at the furthest point along a stem or branch. An axillary bud is located between the leaf and the stem at the node. Axillary buds have the potential to form lateral stems, or branches. Rhizomes are a type of underground stem that travel under the surface and sprout up away from the original plant. Strawberries spread through rhizomes. Even though rhizomes grow under the soil they are not roots.

Leaves grow out from stems and for most plants are their primary site of photosynthesis (the process by which plants convert sunlight into energy). However, some plants, such as cacti, have modified leaves into needles, so most of their photosynthesis is carried out in the stems. Flowers evolved from leaves into the great variety we see today.

Plants have three main types of tissue. Most of these tissues are present in all plant structures, but their function and morphology varies from the roots to the leaves. The three types are ground, vascular, and dermal tissue.

Dermal tissue is all the tissue on the outer layer of the plant. This tissue is responsible for protecting the plant from physical damage and pathogens. Leaf dermal tissue helps regulate the loss of water and the intake of CO₂. Root dermal tissue helps capture water and minerals in the soil. Non-woody plants are covered in an epidermis. In woody plants the epidermis is replaced by periderm in older parts of the plant.

Vascular tissue is responsible for transportation of materials within the plant. The flow usually goes from roots up to shoots, and from the shoots down to the roots. Xylem is the vasculature responsible for transporting water and dissolved minerals and nutrients throughout the plant. Phloem is vasculature responsible for transporting organic nutrients that are produced in the photosynthetic areas of the plant (generally the leaves) and to the roots or sites of new growth.

Ground tissue is all the other tissue in the plant. This tissue is responsible for most plant functions: photosynthesis, storage of nutrients, growth, and support. Ground tissue in the leaves is mostly responsible for photosynthesizing. Ground tissue in the stems, branches, or trunk is responsible for much of the plant support. Ground tissue in tree trunks is often dead and is strictly there to support the outer part of the trunk that is still alive and filled with vascular tissue.

When it’s broken down to the basic level a plant is built from three basic structures: roots, stems, and leaves. These three structures are composed of three types of tissue: ground, dermal, and vascular. These six things make up the basics for almost all plants. That’s amazing when all the variety of plants are considered.