EXUDATIONS AND ROTTING. 235 



cases at any rate, to a conversion of the cellulose, 

 and fungi are sometimes found in the masses of 

 gum ; but beyond the fact that giinimosis is a 

 pathological phenomenon we know very little of 

 the disease. 



With regard to such gumming, it is significant 

 how frequently pruned trees Cherries, Oranges, 

 Lemons, Plums, etc. suffer. 



Manna flux. Certain trees, such as the Manna 

 Ash, species of Tamarisk, etc., yield manna from 

 wounds, and in some cases the latter are due to 

 insects, e.g. Cicada. 



The Potato-disease is best known by the pale 

 whitish fringe, giving an almost meally appearance 

 to the margins of the brown to black patches 

 in damp weather. In dry weather the brown 

 patches shrivel and dry, and as they are apt to be 

 at the edges and tips of the leaflets, these curl up. 

 The young disease spots are )^ellowish, and the 

 leaves of badly affected plants are apt to be sickly 

 yellow throughout. 



This Potato-disease due to PJiytopJithora must 

 be distinguished from the curling and puckering, 

 with wilting and browning of the leaves and yellow 

 glassy look of the stems, due to the invasion of 

 the vessels by a fungus which lurks in the tubers, 

 and gains access thence to the shoots. 



In the disease traceable to Phytophthora the 

 stock remains green and the leaves plump and 

 plane, and only the brown patches slough out 

 in wet or shrivel in dry weather, and are bordered 

 by the pale whitish zone of conidiophores. 



