196 
Functional diseases are due to abnormal activities within the 
nutritive or other chemical processes. The dreaded peach yel- 
lows and the new destructive disease known as “little peach” 
probably both belong here, though their true nature is not yet 
fully understood. The “mosaic disease’’ of tobacco, and the 
“yellow disease” of the china aster are examples of the ab- 
Fic. 26. Witch’s broom found on branches of Picea Muriana (Black spruce) 
from Mt. Desert, Maine, caused by a fungal parasite 
normal production of an enzyme or terment. Diseases of this 
understood. In the case of the “yellow disease’’ of the aster 
the trouble is caused by the failure of the leaf to secrete suffi- 
cient diastase, the enzyme or ferment that converts starch into 
soluble sugars. Starch is being constantly formed in green 
eaves when they are exposed to sunlight, but it is only after be- 
