Summary. In mature plant materials pentosans form the most important food for micro‐organisms. The Klober and Tollens method for the determination of pentosans is not specific for these compounds. While pentosans are easily attacked by micro‐organisms, the other furfurol‐yielding compounds are found to be resistant, and it is therefore essential to get a correct figure for pentosans. A possible method is suggested: to determine the furfuroids in the cellulose obtained by the chlorination method and to deduct this amount from the total furfuroids. Two factors appear to control the decomposition of ripe cellulosic materials in the presence of assimilable nitrogen. The one is the food, or, better termed, energy factor which is the pentosans, the other is the physical or inhibitory factor which is the lignin. It is found that if the ratio of energy factor to inhibitory factor is above 1, the material is easily decomposed; but if it is below 0.5, the material is very resistant to microbial attack. The prediction of the “decomposability” of a material is thus possible. Attempts to increase this ratio in resistant materials by the addition of carbohydrates proved unsuccessful. It was concluded that since micro‐organisms obtained their food materials outside the tissues, they did not attack the tissues until the more easily available food‐stuffs were exhausted. Thus the decomposition of the material was actually less than was possible under natural conditions. Mannose and galactose do not appear to form suitable food for the micro‐organisms concerned in these processes and it is concluded that the pentosan part of the hemi‐celluloses is most important as microbial food. The study of the relative importance of bacteria and fungi proves that under the conditions of these experiments, fungi play a more prominent part especially during the early stages of such decomposition. The study on the availability of the nitrogen of the fungal bodies proves it to be of the resistant type. It seems that at later stages of decomposition under natural conditions fungi are decomposed by other organisms. The ability of certain fungi isolated from such decomposing heaps, to grow at high temperature as well as on purified carbon constituents of plants, and also the presence of almost all the enzymes necessary to hydrolyse the complex carbon constituents, further confirm their importance. The possibility of their activity under natural conditions in manure heaps is strongly suggested.