According to Photosynthesis, what really happened?

Question

A man plants a tree seedling weighing 5 lbs. in a barrel filled with 200 lbs. of soil. He watered the tree regularly. 5 years passed. The tree now weighs 169 lbs. 3 oz. The soil weighs 199 lbs. 14 oz. The tree gained so much weight and the soil lost so little. The man came to the conclusion that the tree gained weight by absorbing the water he had added to the barrel. Why was he misguided? And according to photosynthesis, what really happened?

Answer 1

He wasn't totally misguided, since all living organisms are made up mostly of water.

But the "rest of the answer" that deals with photosynthesis is that the water isn't the only thing the plant took in - there was also carbon dioxide which was combined with the water through photosynthesis to produce glucose molecules.  Besides being an energy source, chains of glucose can be made to produce cellulose, which it the structural component of plants - so the water and CO2 went to producing wood, leaves, flowers, fruit, etc. - anything that contained cellulose.