II 



increasing the thickness of the tree. When the stem is injured, e.g., 

 by animals peeling off the bark, the cambium attempts to grow over 

 the wound and cover it up, but often not quickly enough, if the 

 injury is of considerable area, to prevent wound-fungi, which are 

 among the commonest parasites of stems, obtaining a lodgment. 



The soil is an important factor in the health of the plant. An 

 excess of moisture, not so much as mere moisture but because it 

 excludes air, is highly detrimental ; though the amount which can 

 be comfortably tolerated varies with different species of plant. 

 Equally detrimental is an excess of free acids, especially on a plant 

 which gives off a relatively large amount of water from its leaves 

 (transpires strongly). The roots or trees exercise great selective 

 power, but too much of the mineral salts which are necessary to the 

 healthy growth of a plant may prevent it thriving or set up patho- 

 logical appearances. Though in this as with the amount of 

 moisture what is one species' food is another species' poison. Just 

 as certain species of plants can successfully withstand drought 

 which would be fatal to others, so can particular plants grow on a soil 

 containing a concentration of mineral salts which would stunt or kill 

 those not accustomed to it. There are certain infectious diseases, 

 such as the " mosaic " diseases of tobacco, and ''peach yellows" in 

 the United States, the u spike " disease of sandalwood trees in India, 

 which are not due to fungi or bacteria, but to internal disturbances 

 in the economy of the plant. They are generally assigned to faults 

 in nutrition, to a surplus or deficiency of certain necessary elements 

 in the soil ; but there is considerable diversity of opinion as to the 

 actual causes. 



It has been said, that a plant lives up to its privileges. It 

 endeavours to make arrangements to meet persistent difficulties such 

 as continual strong wind, or a loose soil in which it must make 

 special efforts to attain a firm foothold, but it makes no preparation 

 to meet unaccustomed attacks hence the damage done by storms of 

 wind or hail. If coffee or sugar is cut down before the Para 

 planted among them has got well above their level, it is sure to 

 suffer from wind. When trees are blown down, unless the wind 

 has been much beyond the ordinary, the soil, especially the drainage 

 of the subsoil, and the health of the root must be investigated. 

 When several show a tendency to topple over it is often ascribed 

 to top heaviness, which should be assigned as a cause, if indeed it 

 ever can be one, only when all others have been dismissed as 

 improbable. 



From the foregoing it will be seen that attention must be paid to 

 soil and surrounding condititions and to any unnatural treatment, 

 such as tapping, or cutting leaves, to which a tree is exposed if not 

 alone itself but its neighbours of the same species are to be kept 

 healthy. 



Unhealthiness caused by parasites is the more dangerous. If it 

 assume an epidemic character the monetary loss may be serious, 

 perhaps sufficient to destroy an entire industry. These references 

 are to parasites of a plant nature only (fungi and bacteria). 



