20S PRINCIPLES OF GARDENING. [CH. VIII, 



or to potatoes being grown too often on the same 

 site. It is quite certain from my own experience, 

 that in ground tired of potatoes, the disease invari- 

 ably and most extensively appears. This suggests 

 that it is occasioned by a deficiency of some con- 

 stituent in the soil required by the tubers, a sug- 

 gestion confirmed by the fact that in the fields of the 

 market gardeners near London, which are supplied 

 without stint with the most fertilizing manure, this 

 disease of the potato comparatively is unknown. 



The stems of succulent plants, such as the cacti, 

 mesembryanthemums, and the balsam, as well as 

 the fruit of the cucumber and melon, and the stalk 

 of the grape, are all liable to moist gangrene ; all 

 requiring for the developement of the disease exces- 

 sive moisture in the air, though the immediate cause 

 of its outbreak is usually a sudden reduction of tem- 

 perature. 



Extravasated Sap. — Under tliis general name I 

 puq)ose to include the consideration of gumming, 

 bleeding, and other injurious affections under which 

 plants occasionally labour, on account of their sap 

 escaping from the properly containing vessels. The 

 extravasation proceeds either from the albumnm or 

 from the inner bark, and may arise from five causes : 

 1. The acrid or alkaline state of the sap, which 

 has been considered already, when treating of the 

 canker. 2. From plethora, or excessive abundance 



