Life Science Unit (Book)
Introduction to Plants
Plant Structures
Most modern plants have several structures that help them survive and reproduce. Major structures of most plants include roots, stems, and leaves.
Roots
Roots are important organs in most modern plants. There are two types of roots. First, there are the primary roots, which grow downward. Secondly, there are the secondary roots. These roots branch out to the sides. Together, all the roots of a plant make up the plant's root system. Figure below shows two different types of plant root systems. A taproot system has a very long primary root, called a taproot. A fibrous root system has many smaller roots and no large, primary root.
The roots of plants have three major jobs.
- They must absorb water and minerals, anchor and support the plant, and store food. Roots have special features that are well suited to absorb water and dissolved minerals from the soil.
- Root systems help anchor plants to the ground. They allow plants to grow tall without falling over.
- In many plants, roots store food produced by the leaves. This process is called photosynthesis.
Two types of root systems
Stems
Stems are organs that hold plants upright. They allow plants to get the sunlight and air they need. Stems also bear leaves, flowers, cones, and smaller stems. These structures grow at points called nodes. The stem between nodes is called an internode. (See Figure.) Nodes and internodes of a stem Nodes and internode of a stem
Stems are needed for transport and storage. They carry water and minerals from roots to leaves. It carries dissolved sugar from the leaves to the rest of the plant. The stem is like an elevator for the plant. The stem allows movement from the top of the plant to the bottom and vice versa. Without this connection between roots and leaves, plants could not survive. In many plants, stems also stores food or water during cold or dry seasons.
Leaves
Leaves are the keys not only to plant life but to virtually all life on land. The primary role of leaves is to collect sunlight. This sunlight is needed for the plant to make food. Leaves vary in size, shape, and how they are arranged on stems. You can see examples of different types of leaves in Figure.
Plant leaves come in a variety of shapes and sizes
Variation in plant leaves
Each type of leaf is well suited for the plant's environment. It maximizes light exposure while conserving water. They are also reduces wind resistance. Leaves also benefit the plant in some other way in its particular habitat. For example, some leaves are divided into many smaller leaflets. This reduces wind resistance and water loss.
Leaves are basically factories for photosynthesis.
- A factory has specialized machines to produce a product. In a leaf, the "machines" are the chloroplasts.
- A factory is connected to a transportation system that supplies it with raw materials and carries away the finished product. In a leaf, transport is carried out by veins containing vascular tissue. Veins carry water and minerals to the cells of leaves. They carry away dissolved sugar.
- A factory has bricks, siding, or other external protection. A leaf is covered with dermal cells. They secrete waxy cuticle to prevent evaporation of water from the leaf.
- A factory has doors and windows to let some materials enter and leave. The surface of the leaf has tiny pores called stomata (stoma, singular). They can open and close to control the movement of gases between the leaves and the air.