CAUSES OF DISEASE. 103 



for horticulturists, though it must not be forgotten 

 that no substance is really a food until it is 

 assimilable into the protoplasm : manures, etc., 

 are food-materials, not food. The futility of mere 

 chemical analyses to prove what a plant requires 

 is now well known, and it is only on the basis of 

 long and carefully conducted experiments that we 

 can ever discover what a particular plant in a 

 particular soil, situation, and climate requires for 

 healthy development. Again, the quantity of 

 water in soil may be too great or too small for 

 given species, and this either on the average for 

 the year, or during critical periods only ; and it is 

 obviously important whether the excess or de- 

 ficiency is due to improper supplies of water, the 

 depth or shallowness of the soil, its retentive 

 powers, or the nature of the sub-soil and so on, 

 again bringing the whole matter into connection 

 with our understanding of the physical constitu- 

 tion and structure of soils, and the nature of soil- 

 drainage. 



For instance, a common way of killing ferns 

 is to keep the roots and soil wet and the air and 

 fronds dry, whereas the natural habitats provide 

 for wet and shaded fronds and well-drained soil. 



It may be noted here that in most cases where 

 gardeners speak of plants being killed under the 

 " drip " of trees e.g. Beech, the injury is due, not 

 to the effects of water but to the shade : the loss 

 of light is so great that the shaded plants die of 

 inanition because their leaves are not able to pro- 

 vide sufficient carbohydrates. 



