378 THE NON- VASCULAR PLANTS 



pounds which are very useful to their host. It has been 

 frequently proved that leguminous crops grown on soil 

 rich in the tubercle-forming bacteria do very much better 

 than when grown on soil free from these bacteria. It has 

 been found profitable to inoculate soil with these bacteria ; 

 that is, to introduce them artificially. Due to the assist- 

 ance of these bacteria, leguminous crops are especially 

 rich in nitrogen, and leave the soil in better condition as 

 to nitrogen than it was before they were grown upon it. 

 For this reason they improve the yield of other crops, 

 such as corn or wheat, which may succeed them on the 

 land, and which do not form tubercles. Soy beans are 

 often grown and plowed under for the express purpose of 

 enriching the soil for other crops, and their power to do 

 this is due entirely to the bacteria of their tubercles. 



D. True Fungi. — We speak of true fungi because 

 bacteria are not true fungi. Some of their characteristics 

 make it desirable to class them with fungi, but certain 

 other characteristics make them very different from the 

 true fungi. 



There are thousands of different kinds of true fungi 

 and they have great economic importance. We can de- 

 scribe only a few of the most common kinds. Mushrooms 

 and toadstools are the best known of the true fungi. Per- 

 haps you also know of mold and mildew. They also are 

 true fungi. If you live in the country, you have heard of 

 wheat rust and oat rust, and of corn smut. These plant 

 diseases are caused by true fungi. 



The true fungi have a plant body called a mycelium 

 (plural mycelia). A mycelium is a mass of more or less 

 closely interwoven thread-like structures. These threads 



