34 



BOTANY: PRINCIPLES AND PROBLEMS 



In the case of many species of plants, particularly those which 

 grow in forests or other situations rich in humus, thread-like 

 filaments of fungi are intimately associated with the smaller 

 roots, entering their outer tissues and surrounding the root with 

 a web-like jacket of fungus threads. These very largely take the 

 place of root-hairs and aid the plant in absorbing water and 



Fig. 20. — Root tubercles. A part of the root system of a leguminous pkint, 

 showing the characteristic tubercles upon the roots, caused by the presence of 

 nitrogen-fixing bacteria. 



nutrient material from the soil; and the fungus, as well, is 

 evidently benefited by such relationship. This root-fungus 

 association is known as a mycorrhiza. Certain plants have be- 

 come so dependent in this way upon particular species of fungi 

 that they cannot thrive when these fungi are absent. 



In conclusion, we may emphasize again the extreme complexity 

 of the soil and the vital significance to plants of its composition 

 and of the changes which go on within it. The study of this 

 remarkable material has required the collaboration of almost 



