THALLOPHYTES 61 



(Ascomycete), and that a lichen is simply a sac-fungus para- 

 sitic on Algae. With very few exceptions, all lichens are 

 Sac-fungi. 



40. Soil Fungi. Before leaving the Fungi, further ref- 

 erence should be made to the part they play in the soil. 

 The work of bacteria in the soil was referred to in a preced- 

 ing section ( 37, p. 46), but true Fungi are important 

 also. 



The soil must not be thought of as simply an accumulation 

 of " dirt," but as material very full of life. Just as the waters 



FIG. 48. Section of the cup-like spore-case of a lichen, showing the lining layer of 

 sacs containing spores (h) ; the tangle of the mycelium (m) and the groups of 

 Algse (g) may be seen. - After SACHS. 



are spoken of as " swarming with life," so are the soils 

 " swarming with life," chiefly Fungi. What makes this 

 fact important is that the soil Fungi are important factors in 

 making the soil suitable for plants. The nature of the work 

 accomplished by these soil Fungi, in relation to the use of 

 soil by plants, may be outlined as follows. 



In speaking of the manufacture of proteins ( 29, p. 37), the 

 statement was made that in connection with this process com- 

 pounds containing nitrogen must enter the plant. In the 

 case of soil plants, these compounds are obtained from the 



