MICHIGAN STATE POMOLOGICAL SOCIETY. 477 



soda is in the ash of sea plants. It is found in large quantity 

 in the ash of young and vigorously growing plants,- while 

 it is deficient in those of feeble growth. No land plant 

 can grow in its entire absence, nor maintain a vigorous 

 growth where it is deficient. Soils which contain min- 

 erals rich in potash (e. g., the feldspars) are capable of 

 prolonged cultivation without exhaustion ; hence the strong: 

 agricultural capabilities of clay soils. On the other hand, 

 soils which are formed from materials deficient in potash com- 

 pounds {e. g., silicious soils) are more rapidly exhausted by 

 cultivation ; the vigor of trees growing on such soils is not so 

 great as when growing on soils more richly supplied Avith pot- 

 ash compounds. This is more especially seen in our hardy fruit 

 trees, — the apple and pear. 



Now this deficiency of potash in the ash of the diseased 

 peach tree, as far as it indicates anything, shows impaired 

 vitality, — thus giving a chemical reason for the diminished 

 vigor observed in diseased trees. This impaired vitality does 

 not, in my opinion, for reasons idready given, constitute the 

 contagious Yellows, but it forms a condition exceedingly favor- 

 able for its development, and also a condition under which it 

 is very difficult for the tree to recover when attacked. It is 

 my opinion that not only is the tree enfeebled because it has 

 the Yellows, but it has the Yellows because it was first 

 enfeebled. 



It is well known to physicians that an animal in an enfee- 

 bled condition is much more liable to be attacked by a conta- 

 gious disease than is a vigorous one. The difference in physi- 

 cal condition of a person who has taken a full meal, from that 

 of the same person fasting, may make all the difference between 

 perfect immunity from danger or certainty of attack when 

 exposed to a contagious disease. The same law will doubtless 

 hold good in vegetable as well as in animal life. The condi- 

 tion which will render a vegetable liable to an attack of any 

 contagious disease is a condition of enfeebled vitality. On 



