112 THEORY OF THE DISEASE. 



(443.) The animal lives on the juices of the plant, 

 which it extracts by means of the apparatus which it in- 

 serts into the cuticle of the leaf. This removal of one 

 portion of the sap destroys its proper qualities ; it can no 

 longer return the material for the starch or cellular tissue 

 essential to the growth of the plant. When the growth 

 of the plant is arrested, the natural vital actions are im- 

 paired, and other actions, as those of putrefaction, or inor- 

 ganic changes, take place ; the plant ceases to live in 

 different parts, and decomposition ensues. 



(444.) The exact mode in which death occurs is inti- 

 mately connected with the most obscure functions of the 

 plant. Probably the sap continues to take water from the 

 ground, but in failing to receive sufficient solid material 

 from the leaf, becomes altered and impaired, and thus the 

 plant is killed. 



(445.) The essence of the disease is a disturbance of 

 the relation existing between the leaf and the plant, and, 

 consequently, the sap and parenchyma do not bear such 

 proper relation as fits them for the performance of the 

 vital functions ; and this being the case, the vital func- 

 tions necessarily cease, and the plant dies. 



(446.) The injury inflicted on wild plants is usually 

 confined to the leaf and adjacent stems, the roots, except 

 in very rank growers, being not so much affected. In 

 this way the plant may be gradually killed from above 

 downwards. 



(447.) We find that cultivated plants, where the under- 

 ground stems are highly developed, will not bear the rela- 

 tion between the stem and the root to be interfered with ; 

 and for this reason the same number of insects will exert 



