These transformations of energy involve transformations of matter. The original plant residues may be divided roughly into substances forming the structure of the plant, such as the hemicelluloses, the pentosans, gums, and the contents of the cell--the protoplasm and the storage products, protein; in addition, there are smaller quantities of fats and waxes and other constituents. Some of the easily-decomposable carbohydrates never reach the soil at all, being broken down by intracellular respiration or attack of micro-organisms. But much of the structure material--hemicelluloses, pentosans, etc.--remains. Once the plant residues pass through the earthworm bodies they become completely disintegrated and lose all signs of structure. The only visible product so far known is humus, the black sticky substance characteristic of soil and of manure. Two modes of formation have been suggested. Carbohydrates, sugars, pentosans, etc., are known to yield furfuraldehyde or hydroxymethylfurfuraldehyde on decomposition, and it has been shown at Rothamsted that this readily condenses to form a humus-like body, if not humus itself. In the laboratory the reaction is effected in presence of acid, but even amino-acids suffice. All the necessary conditions occur in the soil, and humus formation may proceed in this way. Some of the structure material--the lignin--contains aromatic ring groupings. Fischer and Schrader have shown that in alkaline conditions these ring substances absorb oxygen and form something very like humus. It is quite possible that humus formation also proceeds in the soil in this way. Whether the two products are chemically identical is not known. The scheme can be represented thus:--