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Cohesion is used to describe the tendency of two of the same thing to "stick together", for lack of a better term, or, at the very minimum, to attract each other with the likelihood that they will stick together. Adhesion is similar, but for two different things. Thus, in plant physiology, consider two water molecules side by side. They attract each other quite strongly through a force called hydrogen bonding. Since one water molecule is just the same as any other water molecule, this force is a cohesive force and cohesion is said to exist between the two. Now, consider a water molecule sitting happily on the outer surface of a leaf, quite happily minding its own business, when it suddenly evaporates due to the air currents over the leaf. As it does so, it pulls the water molecule next to it with it, so that also evaporates. As does the 3rd water molecule, which is cohesively attracted to the second, and so on. You can imagine a "train" of water molecules being pulled into the air. In such a way, an upward flow of water through the plant from the roots to the leaves is achieved, totally against the direction of the force due to gravity. A tension is set up. Try to imagine it as a tug-o-war between the forces that are responsible for transpiration versus gravity, setting up a tension in the "rope" that is the stream of water travelling through the plant capillaries.
Of course, capillary action plays a part. In this, water molecules adhere to the xylem walls. These forces are termed adhesive forces, as opposed to cohesive forces, because water molecules and xylem cell walls are clearly two different things.
I'm sure the websites will describe it in more adult and scientific language, and examiners might want you to do likewise, but I used to find it helped if I could relate it to familiar experiences.
Hope I've helped.
Sorry Qaz. If you note the time of my 2nd post, you may appreciate that I was a little bit under the affluence of incohol at the time. I must resist the urge to post twaddle at such times!
As for my credentials, for what they are worth, I have a B pass at A level biology (1986), though I have to say that plant physiology bored the pants off me! I hope my "explanation" was understandable. Somebody with better credentials than me may want to address the topic in more scientific terms. :)
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