Back To CourseCLEP Biology: Study Guide & Test Prep
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Roots serve many purposes for plants, including the storage of food and the intake of water. Before we look at the structure of roots, let's look at a few examples of roots.
While there are numerous classifications of roots, we will only look at a few. The first one is prop roots. These help brace plants against the elements, such as wind. Corn plants have prop roots as seen here.
Some plants, such as carrots, beets and turnips, have roots that are designed for food storage. These are generally the roots that you may eat.
The last type of root we will look at is designed for water storage. A classic example of this can be seen in pumpkins.
We know that different types of roots have different functions, but different parts of the root also have different functions. Let's now look at a few key components of most roots and relate the structure to the function. While doing this, we will look back at the diagram below of a root in order to better identify the layers. Let's start on the outside and work our way in.
When at a construction site, people are required to wear hardhats in order to protect their heads. In roots, the root cap serves a similar purpose. This outer area of the bottom of the root protects other root tissues as the root continues to grow into the soil. The cells in the root cap are specialized for several different things. First, they can sense gravity, which is why roots grow down. Second, they secrete a slimy substance that helps roots move through the soil.
Directly behind the root cap is the root meristem, which is where cell division occurs. This means that when the root grows, the new cells come from the root meristem. We will look at this structure more another time.
Let's go back to our diagram and label the root cap. You can see that it is found at the bottom tip of the root and forms a protective barrier between the soil and the rest of the root. The small area in orange below is the root meristem where cell division for new growth occurs.
We've talked about the epidermis when we looked at the structure of stems and leaves. Roots also have this protective outer layer known as the epidermis. Remember that 'dermis' means 'skin' and 'epi' means 'outer.' This means that the epidermis is literally the outer skin. While the root cap is found at the bottom of the root, the epidermis is found throughout the length of the root. At the bottom where the root cap protects the root, the epidermis is just inside that hard layer. A good way to relate the epidermis to something more familiar is to think of carrots and potatoes. When you peel carrots and potatoes, you are actually removing the epidermis.
The epidermis can produce root hairs, which are the main site of water and nutrient absorption. Root hairs allow for greater surface area so that plants can take in more vital substances. While root hairs are very important, they only live for a few days. This means that the epidermis must continually produce new root hairs. In some plants, there are even specific types of fungi and bacteria that help with absorption.
Let's go back to our diagram to label these parts. The epidermis is the outer protective layer throughout the length of the root. At the bottom, where we already have the root cap, the epidermis is just inside. Sticking out from the epidermis are a few root hairs. We don't need to label all of them, but do note that there are many root hairs.
As previously mentioned, there are several functions of roots. These include anchoring and supporting the plant, absorbing and moving water and minerals and storing the products of photosynthesis. The primary root tissue is responsible for storing food, which is one of the products of photosynthesis. This layer is sometimes called the cortex. This layer is found after the epidermis and often makes up the bulk of the root. This is especially the case in some plants, such as carrots or radishes. After you peel a carrot, there is a great crunchy snack. The majority of what you are eating when snacking on carrots is the cortex.
The end of the cortex in a root is distinguished by the endodermis. We already know that 'dermis' means skin, so let's consider the prefix 'endo.' 'Endo' means 'inner,' so the endodermis is the inner skin. In roots, this structure contains fatty substances that block water. This means that water will not pass from the inner layers to the cortex and that water in the cortex will not move into the inner layers.
Let's go back and label these structures on our diagram. We can see that directly behind the epidermis is the cortex. This large food storage part of the root is surrounded on the inner side by the endodermis here.
Much like in the plant stem, the vascular cylinder in the root contains xylem and phloem. Xylem moves water and dissolved minerals within a plant, while phloem moves food throughout a plant. The xylem generally moves water up from the roots, while the phloem moves food from the leaves to the roots.
In young roots, the vascular cylinder is in the center. We can see this in our root diagram. Both the xylem and phloem can be found here.
However, in more developed roots, the structure of the vascular bundle may differ. You may remember that there are two types of flowering plants: monocots and dicots. Monocots are flowering plants with one seed leaf, while dicots are flowering plants with two seed leaves. We have been over differences in characteristics in the shoot, or above-ground, system but haven't looked at the root.
In monocot roots, the xylem and phloem are arranged in a ring, much like in the stem. The phloem is on the outside of the ring and the xylem is on the inside of the ring. The ring surrounds the pith in the center. We can see this ring below. Note the phloem, xylem and pith.
In dicot roots, the xylem and phloem are arranged in a star shape. As it is in monocots, the xylem is on the inside of the star and the phloem is on the outside of the star. We can see this arrangement below. Note the overall star shape as well as the phloem and xylem.
Roots serve many important roles in order to help plants survive. The root hairs help absorb water and nutrients from the soil, while the cortex stores the products of photosynthesis, such as sugars. The tip of the root is protected by the root cap and the epidermis provides an outer layer of protection for the rest of the root. In order to move water and food within the root and to the rest of the plant, the vascular bundle contains xylem for the water and phloem for the food. All of these parts must work together regardless of the type of root.
At the end of this video, you should be able to:
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Back To CourseCLEP Biology: Study Guide & Test Prep
23 chapters | 211 lessons