212 Plant Life and Evolution 



the development of active ferments, or enzymes, are 

 able to penetrate the most resistant organic sub- 

 stances, like wood, or the chitinous armor of insects. 

 This ability to destroy organic bodies makes the 

 fungi of some importance in the decomposition of 

 organic matter, although their role in this respect 

 is much less important than that of the bacteria. 



Many of the parasitic fungi, like the black knot 

 of plums, or the onion mildew, cause abnormal 

 growths, sometimes resembling the galls formed by 

 insects. Whether these hypertrophied growths are 

 due to mechanical irritation, or to the effect of en- 

 zymes secreted by the fungus, or to some other 

 chemical stimulus, is not certain. But the abnormal 

 growth is presumably advantageous to the parasite, 

 as the food supply must in this way be notably 

 increased. 



Symbiosis. While most parasitic fungi are very 

 destructive to their hosts, sometimes killing them 

 outright, there is a modified form of parasitism 

 which is of common occurrence. This is known as 

 Symbiosis, and is a phenomenon of much wider 

 occurrence than was formerly supposed. The nitro- 

 gen-fixing bacteria have been referred to in a former 

 chapter, and it now seems certain that a considerable 

 number of fungi can also utilize free nitrogen, and 

 are of material assistance in supplying nitrogen to 

 certain plants with which they live symbiotically. 

 They are often associated with the roots of many 

 flowering plants, especially certain trees like the 



