138 GROWTH AND WORK OF PLANTS 



NUTRITION OF SAPROPHYTES. 



215. Humus saprophytes. Humus is composed of organic 

 matter, largely plant remains in a state of decay. It is abundant 

 on the forest floor and in moors where the decomposition of 

 plant remains is slow and incomplete. As stated in the second 

 paragraph of this chapter, saprophytes are plants which grow and 

 feed on dead or decaying organic matter. Humus saprophytes 

 are those plants which grow in and feed on humus. The humus 

 is not easily soluble, and is rendered available as food for the 

 higher plants by the solvent action of fungi and bacteria. 

 Many fungi, especially the mushrooms, toadstools, puff balls, 

 etc., are humus saprophytes. Many species grow in the forest. 

 The threads of mycelium permeate through the humus carrying 

 on the disintegration which was begun by other species and by 

 bacteria, and appropriating for food some of the dissolved sub- 

 stances. It is by this means that the fungus part of the mycorhiza 

 (see paragraph 206) prepares food for the forest trees with which 

 it is associated. Some of the flowering plants which grow in 

 humus lack chlorophyll, and some or all of their roots are mycor- 

 hizae. These are also regarded as humus saprophytes. The 

 Indian pipe (Monotropa) is one of these. This is a pretty little 

 plant usually growing in a cluster 15 to 20 cm. (6-8 inches) high. 

 It is white in color, sometimes with a reddish tinge. The stems 

 are straight and fleshy, the leaves are scale-like and lack chloro- 

 phyll. The flower in one common species (M. uniflora) is single, 

 and is turned on the stem like the bowl of a pipe. This plant 

 obtains its food through the mycorhiza. The mycelium extend- 

 ing from its surface has the power of dissolving some of the 

 carbohydrates in the humus and passing it over to the mycorhiza 

 so that the plant can be supplied in this way with the carbo- 

 hydrate food which it cannot make from the carbon dioxide of 

 the air because of the absence of chlorophyll. 



216. Saprophytic fungi. The humus saprophytes men- 

 tioned in the preceding paragraph are of course saprophytic 

 fungi, and the term saprophytic fungi applies not only to these 

 humus saprophytes but to all fungi which grow on dead and 



