154 THE BOOK OF NATURE STUDY 



leaf, the nerves or veins. Gradually, very largely owing to 

 bacteria, the softer parts of the leaf which had thus decayed 

 would become food material for large fungi such as toadstools, 

 or for mosses which fruit early in the year, thriving on the decay 

 of the preceding autumn ; these in their turn would decay, and 

 add materially to the leaf-mould, which is often several inches 

 in thickness. It is not only leaves that add to the richness of 

 the soil. Old bark, rotten twigs, covered, it may be, with lichens 

 and mosses, add, in their decay, to the depth of the soil. Even 

 dead logs enrich the soil. Supposing a tree gets wounded, or in 

 any way unhealthy, fungi very soon attack it and destroy the 

 softer tissues, leaving spaces into which insects can easily creep 

 and lay their eggs ; bark beetles, centipedes, woodlice quickly 

 settle themselves in such wood, and thus help on its decay. A 

 fungus which very commonly attacks branches of trees which 

 have been injured in any way is the saddle-back fungus (Poly- 

 poms squamosus). This has fruit bodies of enormous size, 

 sometimes even 2 feet across, and it takes five days for the spores 

 to fall from them. It is easily recognised by its ochre colour and 

 immense size. It does not appear to attack branches or trunks 

 the bark of which is intact. 



Some idea of the decay that goes on in autumn in a wood may 

 be obtained by noting the number of fungi found in it. In the Plate 

 a group of Fly Agarics is seen at the bottom of a birch ; they are 

 very easily recognised by their scarlet colour of the cap, with white 

 warts, and by the ring of tissue attached to the stem, left by the cap. 



The depth of humus, then, is the important factor as far as 

 the soil is concerned. It may be measured by a borer or geotome. 

 This is a stout iron tube, marked in inches, with a handle at one 

 end and a sharp cutting edge at the other ; the tube should be 

 from \ to i inch in diameter. It is fitted with a removable rod, 

 flattened into a disk at one end, and having a handle at the other. 

 This is necessary for forcing out the soil. The geotome should be 

 about 20 to 24 inches long. With such an instrument it is possible 

 to see how deep the surface soil containing the humus is, and at 

 what depth the subsoil begins. It is the decaying organic matter 

 that gives the very dark colour to soil, particularly in woods and 

 forests. This humus also protects the soil against changes of 



