I04 DISEASE IN PLANTS. 



Closely bound up with this is the question of 

 the gases in soils. Apart from the disastrous 

 effects of poisons e.^. coal gas escaping from 

 pipes under pavements in towns, etc., diseased 

 conditions often result from deficiency of oxygen 

 at the root-hairs, due to imperfect aeration of soils, 

 brought about by stagnant water, excess of animal 

 matter, and so forth. 



Unsuitable constitution of the atmosphere is 

 also a fruitful source of disease, though its effects 

 are commoner in closed stoves and greenhouses 

 than in the open. Nevertheless the continual 

 exhalation of sulphurous fumes, chlorine, and 

 other poisonous gases in the neighbourhood of 

 manufacturing centres or of large smoky towns, 

 volcanoes, etc., play their part in injuring plants ; 

 and excessive moisture in the form of mist, rain, 

 etc., is also important. All these matters bring 

 us at once into the region of ph}'siology, and only 

 an intelligent appreciation of what is known about 

 the action of the atmosphere on the soil and the 

 plant will save the peasantry of a country from a 

 hopeless mysticism but little removed from that of 

 the Middle Ages, when blights and other evils were 

 vaguely referred to the river-mists, thunder clouds, 

 and easterly winds. 



If we summarise the above as the material 

 factors of the environment, we may classify another 

 set of external non-living causes of disease as the 

 non-material factors. Such are principally the 

 following : 



The space at the disposal of plants greatly 



