CAUSES OF DISEASE. 105 



affects their welfare. The crowding of roots in 

 the soil and of foliage in the air, resulting in the 

 loss of light to the leaves, involves deficiency of all 

 the materials referred to above minerals, organic 

 materials, gases, and water and no better illustra- 

 tion of the intense struggle for existence among 

 these apparently passive and motionless beings, 

 plants, can be given than an over-crowded seed- 

 bed or plantation. If left to themselves such 

 over-stocked areas exhibit to the keen eye of 

 the trained observer all the phases of starvation, 

 weakness, wounding, rot, and, so to speak, brutal 

 dominance of the stronger over the weaker which 

 it is the object of cultivation to prevent. Here, 

 then, we are brought face to face with the true 

 significance of thinning and weeding out, pruning, 

 and similar processes. 



Unsuitable temperature is one of the commonest 

 of all sources of disease, for every plant is adapted 

 to certain ranges of temperature, and best adapted 

 to a given optimum somewhere between the maxi- 

 mum and minimum temperature for each function. 

 Consequently any serious departure from the mean 

 may bring about physiological disturbances of the 

 nature of disease, and this in very various ways, as 

 exemplified by the results of frost, sun-scorching, 

 drought, hail-storms, forest fires, and so forth. 



As a predisposing factor to disease abnormal 

 temperature effects play a great part. Many 

 wound -fungi gain their entrance through frost- 

 cracks, bruises due to hailstones, or into tissues 

 chilled below the normal. 



