The term 'Tree Substance' can be summarized by the crossword solution 'SAP'. SAP stands for 'SAPWOOD', which is the soft, spongy layer of wood found between the bark and the heartwood of a tree. It is composed of living cells that transport water and nutrients from the roots to the leaves. SAPWOOD is essential for the growth and health of a tree, and it is often used in the production of lumber and other wood products.