Plant parts and how a plant grows

Parts of the plant body

All plants have roots, that are usually belowground, and shoots, that are usually aboveground and include stems, leaves, and (sometimes) flowers.

Below is a good diagram of basic plant structure, adapted from the Arizona Master Gardner Manual (produced by the Cooperative Extension, College of Agriculture, University of Arizona, http://ag.arizona.edu/pubs/garden/mg/botany/plantparts.html). I have revised the image slightly by changing one label to read "Axillary bud".

At the tip of the stem is the terminal bud. This is where growth occurs, producing new leaves and new stem tissue.

Sections of stem that are left behind as the stem tip elongates may be classified as nodes (sections of stem to which leaves are attached) and internodes (the regions between the nodes). Nodes may or may not be swollen.

Leaves may have two distinct parts: the flat blade and a stalk called the petiole. The thin, flat shape of most leaves maximizes surface area for absorbing light and carbon dioxide for photosynthesis. Variations in leaf structure are discussed below.

The upper angle between the leaf and the stem is called the leaf "axil". Think of the axil as an upside-down armpit. In that axil is an axillary bud. An axillary bud is just like a terminal bud. It may grow out to produce a branch, which has all the same parts as the main stem (nodes, internodes, leaves, etc.). It may grow out to produce a flower, or it may never grow.

We will discuss flower structure in a later lesson.

Roots have root caps at their tips: a cap of tissue that protects the tender, growing young tissue as it pushes through the soil. Roots produce root hairs a short distance behind the root tip. These are hair-like extensions of single cells that increase the surface area of the root for absorbing minerals and water. Lateral roots that grow out from the primary root have the same parts as the first, or primary, root.

This diagram also shows internal vascular tissues. Briefly, plants have two types of vascular tissues inside them: xylem, which transports water and any minerals dissolved in water, and phloem, which transports sugar. Xylem contains chains of dead cells with stiff cell walls that act as pipes or soda straws, allowing water to be drawn up the plant from the soil as water evaporates from the leaves. This is very similar to the way in which liquid is sucked up a soda straw. The mechanism driving sugar transport in the phloem is more complex. Suffice it to say that the phloem can transport a sugar solution from leaves, where sugars are produced by photosynthesis, to anywhere in the plant where sugars are needed to provide energy or materials to grow new plant tissue. It also can transport sugars from storage organs (like swollen underground stems or roots), where carbohydrates are stored as starch for later use, to places in the plant where energy and/or carbon is needed to grow or maintain plant tissue.