506 SCIENCE PROGRESS 



kilogrammes of nitrogen per hectare for foliage-production, 

 and that 1 1 kilos, were utilised in wood-production, while 

 the total amount of nitrogen contained in the atmospheric 

 precipitations amounted only to u kilos., or merely enough for 

 the wood-production. The deduction drawn from this was that 

 the leaf-mould was, directly or indirectly, instrumental in pro- 

 viding the further supply needed ; and that therefore, as long 

 as the wood was kept close and the dead foliage retained on 

 the soil, there would be no fear of the necessary supply of 

 nitrogen running short. Further light was thrown on this 

 subject by the researches of Frank, Moller, Wollny, Henry, 

 and others, showing that on the roots of many woodland trees, 

 and especially the Cupuliferae and Coniferae, a growth of fungi 

 (mycorhisa) takes place, which enables the humus, and particu- 

 larly the nitrogen, to be utilised as food not otherwise available. 

 While the humus is in process of decomposition, carbon 

 dioxide, water, ammonia, and free nitrogen are set free and 

 assume forms making them available as plant-food. 



According to the stage of decomposition at which it has 

 arrived, the cast foliage in woodlands may be classed as (i) raw 

 (non-decomposing) humus, (2) coarse (partially decomposed) 

 leaf-mould, and (3) fine (decomposed) leaf-mould, the first 

 being formed under unfavourable conditions, and the other 

 two under favourable conditions — though all of them are 

 formed under conditions different from those obtaining on 

 unwooded moors or peat-bogs, where the dead portions of 

 the plants growing there form a peaty layer of greater or less 

 depth, owing to want of aeration in the soil. But it is now 

 generally accepted as fully proved that the decomposition of 

 dead foliage into leaf-mould is not purely a chemical, but a 

 chemico-physiological process, in which micro-organisms play 

 a very important part; for without their aid neither the with- 

 drawal of carbon dioxide from the decomposing leaves, nor 

 the formation of ammonia, nor nitrification, nor denitrifica- 

 tion is possible. But the composition and the physical and 

 chemical properties of humus are more or less dependent 

 on the stage to which decomposition has advanced under the 

 given conditions obtaining throughout the soil ; and in course 

 of time the humus gradually disappears on its organic substance 

 being dissolved into carbon dioxide, water, and ammonia, 

 transformable into nitric acid, while the ultimate mineral 



