SECT. II. DISEASES. 49? 



Sometimes the disease is occasioned by the too 

 rapid growth of a particular branch, depriving the 

 one that is next it of its due nourishment, and 

 hence inducing its decay. Sometimes it is occa- 

 sioned by means of parasitical plants, as in the case 

 of the bulbs of the Saffron, which a species of 

 Lycoperdon often attaches itself to and totally cor- 

 rupts. The harmattan winds of the coast of Africa 

 kill many plants, by means of inducing a sort of 

 gangrene that withers and blackens the leaves, and 

 finally destroys the whole plant. The Nopal of 

 Mexico is also subject to a sort of gangrene that 

 begins with a black spot, and extends till the whole 

 leaf or branch rots off or the plant dies. 



But plants are sometimes affected with a gan- Wet gan- 

 grene by which a part becomes first soft and moist, 

 and then dissolves into foul ichor. This is confined 

 chiefly to the leaves, flowers, and fruit. Sometimes 

 it attacks the roots also, but rarely the stem. It 

 seems to be owing in many cases to too wet or too 

 rich a soil ; but it may originate in contusion, and 

 may be caught by infection. 



But the Nopal is subject also to a disease called Dissolu- 

 by Thiery la dissolution, and considered by Sir J. E. 

 Smith as distinct from gangrene. I cannot however 

 perceive the difference ; I think it is Willdenow's 

 dry gangrene. A joint of the Nopal, or a whole 

 branch, and sometimes an entire plant, changes in 

 the space of a single hour from a state of apparent 

 health to a state of putrefaction or dissolution. 



VOL. II. 2 K 



