Subject: How does sugar water affect bean plant growth?

I'm doing a science experiment on sugar water and beans. I 
observed that the bean plants watered with the sugar solution 
tended to stay moist longer than the normal water. Why does this 
happen?

PS. I have the beans in (forgive my US measurements) 1.5 cups of 
potting soil. I water them with 1/3 cups of water for each 
plant. On one group, I add 2 teaspoons of sugar to the 1/3 cup 
of water, in anoter group I add 1 teaspoon, and I use normal 
water for the last group.

Re: How does sugar water affect bean plant growth?