Carbon is stored inside plants as carbohydrates. 

Plants take in carbon dioxide from the atmosphere through stomatal pores in their leaves. Inside, the carbon dioxide combines with water to form carbohydrates. These are transported to various tissues of the plants like stem, roots, fruits etc. and stored as food that can be used for their growth.

Different types of carbohydrates can be found inside plants. Initial steps of carbohydrate synthesis forms phosphoglycerates, glyceraldehyde phosphates, malates etc. 
The stored form of carbohydrates include cellulose, starch, sucrose, fructose, glucose, maltose etc.

Half of the dry weight of wood is carbon in the form of carbohydrates like cellulose, hemicelluloses like xylan, pectin etc.