206 COLLEGE BOTANY 



are some species which tend to reduce the amount of nitrogen in 

 the soil, still others cause diseases of plants and frequently cause 

 heavy losses. Among the most interesting are the mycorrhizal 

 fungi which are found associated with the roots of many 

 higher plants. 



They grow in and on the roots, forming a structure which 

 absorbs the soil water very readily. In most cases they are 

 considered as being beneficial, and some plants, such as the blue- 

 berries and the Indian pipe, will not grow if the fungus is not 

 present. In some few cases they are believed to be injurious. 

 Many of the large fleshy fungi (toadstools and mushrooms) pro- 

 duce mycelium growths in the soil ; in most cases they are 

 saprophytic, but some of them are parasitic on the higher plants 

 and the cause of losses in crop production. Some of the very 

 small green algae (see page 260) grow in the surface strata of 

 moist soil, but their influence on the soil is not well understood. 

 Some soils contain great numbers of unicellular animals, known 

 as Protozoa. They feed on bacteria, and it is very generally be- 

 lieved that their presence is injurious to the soil. 



Water Cultures. Watei cultures are very generally used to 

 demonstrate the nutrition requirements for plants. They con- 

 sist of mineral solutions with seedling plants suspended at the 

 surface, so that the root systems will be submerged. The most 

 common solutions are : 



1. Pfeffer's Solution. 



Calcium nitrate 4 grams 



Potassium nitrate 1 gram 



Magnesium sulfate 1 gram 



Potassium dihydrogen phosphate 1 gram 



Potassium chloride 5 grams 



Ferric chloride trace 



Distilled water 3 to 7 litres 



