244 



A TEXTBOOK OF BOTANY 



[Ch. V, 6 



importance, of the soil constituents are certain minute liv- 

 ing organisms, viz. Fungi, Bacteria, and Protozoa. 



Fungi, of certain small kinds, develop in contact with the 

 tips of the roots of many plants, particularly such as live 

 in much humus, weaving around them a close cover of my- 

 celial threads, which replace the root hairs (Fig. 172). This 



mycorhiza, as it is named, ab- 

 sorbs water and mineral matters 

 which it transmits to the roots; 

 and there is some reason to be- 

 lieve that it also absorbs solu- 

 ble organic matters set free in 

 decay of the humus but useful 

 again to the plants. The associa- 

 tion seems clearly beneficial both 

 to fungus and flowering plant; 

 and accordingly we have here 

 one of the cases where two dif- 

 ferent organisms derive benefit 

 from their association, a condi- 

 tion called symbiosis. Some 

 kinds of soil Fungi seem also to 

 have the same powers as Bacteria, 

 next described, in relation to soil 

 nitrogen. 



Bacteria, already known to the 

 student as the smallest and 

 simplest of living organisms, are abundant and of many kinds 

 in all soils ; but the most important are those which effect 

 nitrification and nitrogen fixation. Nitrogen, a con- 

 stituent of the protoplasm, is one of the substances most 

 indispensable to plants; but although it composes four 

 fifths of the atmosphere, the higher plants are unable to take 

 it from that source, and have to rely upon compounds ab- 

 sorbed in solution through the roots. The presence of 

 mineral salts containing combined nitrogen is therefore one 



Fig. 172. — Typical Mycorhiza, 

 on the root of European Beech ; 

 X 120. 



The entire root tip, back to 

 beyond the hair zone, is com- 

 pletely and closely covered by a 

 felted mass of mycelial threads, 

 which extend also into the soil. 

 (After Frank and Tschirch.) 



