That's a good question and would be a cool experiment to do in school. It's amazing what plants can do! There are two cells called guard cells that are on the outside of each stoma. These cells control the opening and closing of the stomata. They way they work is by either taking in water or releasing water. When they take in water, they swell up. This swelling causes the two cells to bend, which in turn opens up the pore. When they lose their water and return back to normal size, they are no longer bent and can lay side by side, closing the pore. If a concentrated solution of glucose is placed on top of a leaf where the stomata are, it will cause water to come out of the guard cells, making the stomata close. I've included a picture showing what open and closed stomata look like.picture-click here
Glucose, being dissolved in water, has the effect of causing the solution it's in to suck water out of other things in order to dilute the solution. Plants lose water through their stomata, so if a plant is having water sucked out of it by the glucose solution, it doesn't want to lose any more water through its stomata.
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