DEVELOPMENT OF PLANTS 



219 



will doubtless be greatly improved. It contains bacteria, and if 

 not fresh will seriously interfere with the quality of the bread. 

 It should be added that some bacteria are beneficial since by 

 their presence they exclude injurious forms. Possibly they also 

 cooperate with the yeast in producing substances in the dough 

 that impart to bread its agreeable flavor. Certain it is that 

 dough raised by blowing air into it does not make as pleasantly 

 flavored bread as when the dough is raised by yeast. 



84. Order b. Exoascales or Peach Curl. — This group includes 

 a small number of parastic fungi that are especially destructive 

 to peach and plum trees, causing distortion of the leaves and 

 fruit, known as leaf curl and bladder plum (Fig. 140). Numer- 



FiG. 140. 



Fig. 141. 



Fig. 140. A branch from peach tree, showing the distortion of the leaves 

 caused by the fungus, Exoascus. 



Fig. 141. Section of a leaf showing numerous cells rupturing the cuticle 

 and developing into asci, as. 



ous asci are developed from the mycelium beneath the cuticle, 

 which is finally ruptured by their growth (Fig. 141). The asco- 

 spores are discharged into the air by the bursting of the asci and 

 carried by the wind to other plants. The damage in the United 

 States to peach trees alone is estimated at $2,000,000 to $3,000- 

 000 annually. 



85. Order c. Aspergillales or Blue-green and Brown Fungi. — 

 This group includes perhaps the most widely distributed and 

 familiar examples of the fungi. They occur as blue-green or 

 brown moulds upon almost any organic matter, forming a deli- 



