58 the president's address. 



in the cell-sap of a substance which to that particular fungus 

 is negatively chemotactic. Thus it comes to pass, that those 

 fungi whose germ-tubes are attracted by substances present in 

 the cell-sap of a given plant, can enter its tissues and become 

 parasitic upon it ; whereas, on the other hand, those fungi 

 whose germ-tubes are repelled by a substance present in the 

 cell-sap cannot enter the living tissues. 



It has been shown that certain bacteria are influenced by 

 chemotactic substances in the same manner as fungi. 



It is well known that very different substances are normally 

 present in the cell-sap of different parts of a plant. This 

 probably accounts for the fact that certain fungus parasites 

 are restricted to different portions of the structure. For 

 instance, some fungi are parasitic in the anthers only, others 

 are confined to the foliage, wood, or roots respectively. 



In the distribution of fungus spores, wind must be perhaps 

 accorded the first place, although it has been proved in the 

 instances of several serious epidemics that insects are the main 

 agents in carrying spores from one plant to another, and in 

 some fungi special arrangements are present for aiding this object. 

 The most general contrivance of this kind is where the spores 

 of the fungus are imbedded in some sweet substance eaten by 

 insects. In this case the spores pass uninjured through the 

 body of the insect, and are thus distributed. 



With these few remarks bearing on the general subject of 

 parasitism, we must now pass in rapid review a few of the 

 most frequent diseases of plants caused by fungi. 



Peach Leaf-curl. 

 (Exoascus deformans.) 



This parasite attacks the young shoots and foliage, and is 

 recognised by the distorted and curled leaves, which are at 

 first pale green and afterwards tinged red, and covered with 



