The xylem elements perform the function of transport but they also help to support plants...

Explanation

The xylem is a type of plant tissue that is responsible for transporting water and minerals from the roots to other parts of the plant. However, in addition to its transport function, the xylem also provides support to the plant. The question asks why this is the case, and presents four possible options to choose from. Option A suggests that the xylem is located internally, Option B suggests that it is tubular, and Option C suggests that it has rigid thick walls. However, the correct answer is Option D, which states that the xylem constantly absorbs water. The absorption of water by the xylem causes it to become rigid and stiff, providing support to the plant. This is because water molecules are attracted to each other and tend to stick together, a property known as cohesion. As water is absorbed into the xylem, the cohesive forces between water molecules cause the xylem to become turgid and rigid, which in turn helps to support the plant. It's important to note that while all of the options presented in the question may have some relevance to the xylem's role in supporting plants, only Option D is the correct answer. To learn more about this, please read the relevant sections of the recommended textbooks.