PLANT-LIFE IN HUMUS 355 



and Toadstools (Fig. 226). The threads (or hyphae) of 

 the mycelium obtain their food from the dead leaves and 

 other organic matter ; and, along with organisms like 

 bacteria, are responsible for their decay. The Toadstools, 

 therefore, live upon the complex organic substances of 

 dead leaves and twigs, or on animal remains and excre- 

 ments ; whereas the food of green plants consists of 

 solutions of certain mineral salts and carbon dioxide. 

 As there is much available energy in the organic compounds 

 in humus, Fungi, by making use of this, do not need to 

 absorb light-energy, and can thrive without developing 

 either leaves or chlorophyll. 



Saprophytes, Mycorrhiza, and Symbiosis. Plants growing 

 upon dead organic matter (animal or vegetable) are called 

 saprophytes (Gr. sapros = rotten). Some fungal sapro- 

 phytes are very restricted in their distribution, and occur 

 only on the fallen leaves of particular species of trees. 



You will find mycelia abundant in the damp leaf-mould 

 of woods, and the roots of plants growing in the mould 

 often become intimately surrounded by hyphal threads. 

 Sometimes the hyphae enter the tissues of the roots and coil 

 up within their cells (Fig. 229 a). They gain an entrance, 

 not merely by mechanical pressure, but also by means of 

 a ferment which they secrete and which digests the cell- 

 walls of the roots with which they come into contact. 

 Usually they do no harm to the roots, and possibly they 

 convey to them useful materials from the humus. 



Fungi thus do an important work ; they decompose the 

 cast-off leaves, prevent their accumulation, and convert 

 their constituents into useful food for green plants. 

 They may even convey this food into the tissues of the 

 plants. In return they receive shelter and possibly some 

 food from the roots. 



Such a combination of fungal hyphae and the roots of 

 plants is called a mycorrhiza (Gr. mykes - a fungus, 



z 2 



