DISCUSSION OF DEPENDENT PLANTS 379 



inserted into a new twig, leaf, or floral structure, the infec- 

 tion may spread several inches and soon the blighting begins. 

 When one flower is infected, insects may carry the bacteria 

 to practically every flower upon the tree or upon other trees 

 within the vicinity. Moreover, when the disease has developed 

 far enough for the characteristic gummy exudations to appear, 

 insects that bite into them may become loaded with the bac- 

 teria and may insert some of them into a new host. In prun- 

 ing both diseased and healthy twigs, the knife may be the 

 means of transferring bacteria. If all infected parts are re- 

 moved and burned, and if the knife used in pruning diseased 

 twigs is sterilized before being used in pruning healthy plants, 

 the continued spread of the disease is made unlikely. 



The " pear-blight " bacteria, therefore, are dependent upon 

 such plants as the apple and pear for nutrition, dependent upon 

 their twigs for protection through the winter, and dependent 

 chiefly upon insects for distribution to new hosts. This is 

 but one of many illustrations that might be cited to show the 

 degree of dependence to which parasitic bacteria have come. 



345. Other saprophytic fungi. One has but to observe care- 

 fully in any deeply shaded, moist, and warm undergrowth to 

 see abundant illustrations of molds, mushrooms, and toadstools 

 which are dependent upon decaying organic matter. Usually 

 the major portion of such a saprophytic plant lives within the 

 supporting substance and gathers nourishment from it. In 

 this way the saprophyte may live for months or years with no 

 external appearance of its growth. Upon breaking open an old 

 log or stump, or upturning the soil, one often finds the exten- 

 sively interwoven network of a saprophytic fungus. After a 

 period of growth by the mycelium of the saprophyte, its super- 

 ficial reproductive structure develops under favorable condi- 

 tions as a sort of final and outward expression of its previous 

 more or less prolonged period of nutrition. Spores thus pro- 

 duced may be carried by agencies such as the wind, insects, or 

 other animals. When they fall in favorable locations they ger- 

 minate, penetrate nutrient substances, and continue the life 



