218 BOARD OF AGRICULTURE. 



Afternoon Session. 



The Board met at two o'clock ; and Professor W. G. Fae- 

 LO"W of Cambridge read the following paper : — 



DISEASES OF FRUIT-BEARIXG TEEES. 



BY PEOFESSOE W. G. FAELOTV. 



No thorough knowledge of the diseases of plants can be 

 obtained without first making a study of their microscopic 

 structure, and of the chemical and vital relations which 

 their different organs bear to one another. On the present 

 occasion, however, it is out of the question to consider these 

 points in detail. It will suffice, in this connection, to say 

 that all plants are composed of cells, or sacs, filled with semi- 

 fluid contents. Sometimes the cells elongate so as to form 

 filaments, and in some cases their walls become thickened 

 and hardened. The life of a plant lies principally in the thin- 

 walled cells, which are spherical, or as nearly spherical as 

 their contact with one another will permit. In studying the 

 diseases of plants, we must always ask ourselves the question, 

 What change has taken place in the cells themselves ? for it 

 is only by examining the changes which the individual cells 

 have undergone, that we can, in most cases, understand the 

 deformities and changes, which, from their size, are conspicu- 

 ous even to the most careless observer. 



A disease may be strictly defined as that which causes 

 temporarily or permanently a disturbance of the function of 

 any part of a plant or animal. The causes of disease in plants 

 are principally insects, fungi; and to these we may add cer- 

 tain spontaneous changes in the plant-cells themselves. Of 

 these three causes, without doubt the greatest harm is done 

 by insects. Next in frequency and importance come diseases 

 caused by fungi. Of the third class, caused by changes 

 which arise spontaneously in the plant-cells, but little is 

 known ; and undoubtedly some of the changes to which we 

 now give the name of spontaneous — thus conveniently cover- 

 ing our ignorance as to their real character — will hereafter 

 be found to originate in some definite external cause. What- 

 ever be the cause of a disease, whether insect or fungus, its 



