Can someone explain to me how the trunk of the boab tree conserves water?

The secret of the baobab's success in surviving in harsh environments and the reason for its mive trunk is that it has little wood fibre but a large water storage capacity. Each tree can hold up to 300 litres of water, enabling it to live through long periods without rain. Their life-cycle is as impressive as their bulk - most live over 500 years and some specimens in Africa are believed to be up to 5000 years old