ADDRESS BY THE PRESIDENT, NOVEMBER 3, 1875. 13 



mingham Town Council has voted £1000 for the purpose of 

 planting trees in the open spaces of that town, and protecting them 

 with guards. It is difficult for trees to grow well in the midst 

 of dwelling-houses, unless space is allowed, and unless means are 

 taken for consuming smoke, especially in the case of mills and 

 manufactories ; and of preventing the issue of noxious vapours, 

 such as sulphurous acid and hydrochloric acid. The injury done 

 to trees by alkali works is very great. This subject was investi- 

 gated many years ago by Sir Robert Christison and Professor 

 Turner of London, and they found that a very small amount of 

 these gases in the atmosphere, even so little as 9 \ , 6 part, injured 

 the leaves of trees, and made them fall off. 



A subject of great importance, and which must necessarily 

 engage the attention of foresters, is the causes of disease and decay 

 in forest trees. This subject has been strongly brought under notice 

 by the falling off in the Larch, and other forest trees. The most 

 common causes of disease are, — improper soil, ungenial climate, 

 frosts, long-continued rains, great drought, violent storms, parasitic 

 plants, insects and worms. 



The diseases of plants may be divided in the following way : — 

 1. Diseases which are caused by an excess or deficiency of those 

 agents which are necessary for the vigorous growth of plants ; 

 such as soil, light, heat, air, and moisture. 2. Those which are 

 either originally caused, or, at all events, aggravated and modified 

 by the attacks of parasites, more particularly belonging to the 

 natural order Fungi. 3. Those clue to the action of poisons, either 

 taken up from the soil or from the atmosphere. 4. Those caused 

 by mechanical injuries of different kinds, as by the attacks of 

 animals, more particularly insects. Diseases caused by changes 

 in the atmosphere are often epidemic, and spread over extensive 

 districts of country. Those which are due to parasitic fungi are 

 propagated by contagion — the minute spores being carried by the 

 winds. Exciting causes operate with great intensity in cases 

 where plants are previously predisposed to disease. Thus, if a 

 plant is in an enfeebled or weak condition, it is very liable to 

 suffer both from epidemic and contagious diseases. 



The cryptogamic diseases of plants must be considered con- 

 tagious, since they are produced by the contact of one portion of 

 organic matter with another. The contact of diseased cells pro- 

 duces disease in healthy cells. The action is analogous to what 

 takes place with ferment when introduced into a saccharine liquid. 



