196 HIGHER PLANTS AND SOIL MICROORGANISMS 



organisms and higher plants than those already mentioned. In 

 such cases, as with the hchens where there is associative develop- 

 ment of algae and certain higher fungi, definite dual plant forma- 

 tions develop which appear Hke single plants. The fungus derives 

 organic nutrition from the material synthesized by the alga, while 

 the latter makes use of inorganic nutrients absorbed by the fungus. 

 Such an association permits the lichens to develop under condi- 

 tions almost prohibitive to growth of either of the associates 

 alone. 



Under conditions where some of the single-celled algae grow in 

 association with nitrogen-fixing bacteria, the latter develop at the 

 expense of some of the organic matter formed by the algae and 

 fix nitrogen during their growth. 



Mycorrhiza. — Various symbiotic and parasitic associations 

 are known with the higher plants. Penetration by certain micro- 

 organisms of roots of most plants, including annuals and perennials, 

 woody plants, and herbaceous plants, is probably the rule rather 

 than the exception. Roots of all plants growing in soil are liable 

 to invasion by any fungal hyphae which can effectively penetrate 

 them. Some of the fungi attack the roots and destroy the vege- 

 tative tissue. The type of development depends upon the nature 

 of the parasite, resistance of the plant, and conditions favorable 

 or unfavorable to the parasite and host. 



In some cases, however, the host plant is able to overcome the 

 attack by the fungus and may even benefit from it. An associa- 

 tion may thus be established, which is referred to as mycorrhiza, 

 derived from the words viyces = fungus, and rhiza = root. This 

 formation is a very widespread and common phenomenon among 

 plants. It may prove to be of considerable importance in plant 

 growth, affecting it favorably or unfavorably. The extent of the 

 penetration of the fungus and degree of its effect upon the growth 

 of the plant vary with the soil conditions, which determine the 

 plant vigor and degree of its resistance. Under favorable cultural 

 conditions, there may be no apparent injury to the plant, but under 

 adverse conditions of moisture, temperature, or nutrient supply, 

 the fungus may become a parasite upon the plant and cause 

 definite injury. 



The nature of mycorrhiza produced on various plants, as well 

 as their role in the nutrition of the plants, differs with different 

 plants. In some cases, the fungus does not penetrate into the 



