194 ON THE EFFECTS PEODUCED ON TREES AND SHRUBS 



Probably the most fatal enemy to plant-life is an acid 

 vapour termed liydrochloric acid (a mixture of hydrogen and 

 chlorine). It is evolved among the dense white fumes issuing 

 from most chemical works, and in a very short time causes 

 absolute sterility to all land within its influence. Its poisonous 

 effects upon a tree or shrub are first observed by the shrivelling 

 up and drooping of the leaves, which in a few days are shed. 

 The young wood buds lose their plump, healthy aspect, becoming 

 scaly and falling off; the twigs then present a dead appearance, 

 which soon spreads itself down the branches, the bark ci^acks 

 and shrivels off, here and there on the stem, a young bud may 

 in spring be seen struggling in the vain effort to develop itself 

 into a new branch to sustain the failing vitality, but very soon, 

 these last attempts at self-resuscitation cease, and the tree dies. 



The effects produced upon tree and shrub life by smoke from 

 public works are similar in operation to those now described, 

 and when the functions of the leaves in vegetable economy are 

 considered, it will be easily understood how the health of plants 

 is interfered with by these effects. 



As the function mainly of the foliage of a tree or shrub, is to 

 expose the secretions of the plant to light and air, and so to 

 assist in the formation of wood, if the leaves are coated over by 

 dense deposits of carbon and other substances, a film is formed 

 sufficiently opaque to prevent the free action of light and air 

 upon the leaves, and hence the secretions necessary for the for- 

 mation of wood-buds are impeded in their development. Hence 

 we see how stunted in stature, and in thickness of trunk, are trees 

 in the vicinity of towns as contrasted with others of similar age 

 planted in the open country, or in rural districts with an uncon- 

 taminated atmosphere. 



Again, leaves have another most important function to perform 

 in the healthy economy of trees and shrubs. They daily give off 

 into the atmosphere large quantities of watery fluid by the 

 process which they perform, called in technical terms exhalation. 

 The importance of this function in relation to smoke deposit on 

 the leaves, must be at once apparent. The moisture on the sur- 

 face of the leaf, created by its healthy action, at once attracts the 

 minute particles of carbon and other injurious ingredients which 

 thereby adhere to tlie leaves, and, coating them over, at once 

 arrest the healthy process of exhalation over the whole system 

 of the plant; for as the leaf .surface cannot find vent for the 

 secretions pumped up from the root action, this latter function 

 is next impaired, and a drag, so to speak, is put upon its healthy 

 flow. The humidity of air which trees promote in their imme- 

 diate proximity, by the process of exhalation, is very materially 

 lessened by its cessation, and the moist and cool condition of 

 soil, which is so essential to their welfare, is in its turn affected, 



