Answer:
Cohesion is the forces holding water molecules together. This concept was covered when talking about properties of water and capillary action. You may remember capillary action is like when you have a little bit of water in the bottom of your cup but the water in the straw is a little bit higher than the water in the cup. Because water is polar - meaning it has a somewhat negative end and a somewhat positive end - it is attracted to other water molecules and other substances, such as the straw.
Cohesion
Now, you may be asking yourself: what does a straw have to do with xylem? Well, just like the water moving up the straw, water moves up xylem. The movement of water in plants is from the roots up through the shoot and out the leaves. Part of the reason for this movement of water is cohesion. The water molecules within the xylem tend to stick together, which allows them to help pull other water molecules up through the xylem - even against the flow of gravity.
When water is moving through the xylem, it is moving between different vessels or tracheids. It is very important that these elements are tightly held together because if there are air bubbles, the effects of cohesion are diminished. That is, if air bubbles get into the xylem, the water will no longer move up through these tubes. We can relate this idea back to our straw. If you have ever had a straw that has even a tiny hole in it, you know that it is very hard - even with the aid of suction - to get the water through the straw and up to you. This is what happens if air bubbles get into the tubes of the xylem.
So , indirectly cohesion helps in farming through xylem....
