216 



Presence 193. The presence of poisonous 



Substance ** substances in the air may cause disaster to plants just 

 as may their presence in the soil. More than 4 per 

 cent, of carbon dioxide in the air is injurious to many 

 plants. Sulphur dioxide and fumes of hydrochloric acid are 

 especially harmful, to which fact is to be attributed the injurious 

 effect of an excessive amount of coal smoke, such as occurs in 

 many towns, and of the fumes from iron-smelting furnaces, 

 alkali and other chemical works. The foliage of plants thus 

 injured usually turns yellow, and the plants become sickly and 

 may eventually die. Trees usually suffer more than herbaceous 

 growth and forests have been injured at a distance of 4| miles 

 from the place of origin of the poisonous fumes. The Plane 

 (Platanus) is able to withstand the injurious effect of coal smoke 

 and grows well in smoky cities, as do also as a rule maples, 

 horse-chestnut and elms. 



Effect of 194. The action of lightning 



Lightning. i n causing the death or disease of plants is not clearly under- 

 stood. In some cases a tree struck by lightning remains 

 practically uninjured ; a narrow strip of the cortical 

 tissues is separated from the wood and the wound is soon 

 occluded. In other cases, where the external signs of injury 

 are similar, the entire tree may die. Stems of trees in some 

 cases may be completely barked by lightning, in other cases they 

 may be shattered into fragments and occasionally set on fire. 

 The injurious effects may be confined to a single tree, or the 

 lightning, passing from the stem originally struck to others near 

 it, may damage a large group of trees. In the latter case the 

 extensive damage appears to be often due to the lightning 

 passing from root to root in the ground owing to the bad con- 

 ducting nature of the soil. In such a case the injury spreads 

 from a certain point in a centrifugal direction. Trees injured 

 by lightning may sometimes remain alive for 4 or 5 years and 

 then die. 



SECTION V. EFFECT OF FIRE ON PLANT-DEVELOPMENT. 



195. The injurious action of fires 



consists in destroying leaves, flowers, seeds, twigs, branches, 

 seedlings and young plants, and in scorching, and more or 

 less extensively injuring, the roots, cortex and cambium of 

 older trees. 



Moreover, the soil may be injuriously affected owing to the 

 destruction of humus and to a decrease in the amount of avail- 

 able moisture. Fires may thus be responsible for wounds and 



Effect of 



