122 DISEASE IN PLANTS. 



such want or irritation be due to sonne defect in 

 soil or air, or to the ravages of a fungus or an 

 insect. 



This being understood I need not dwell on the 

 common fallacy of confounding the fungus, insect, 

 soil or other agent with the disease itself, or of 

 making the same blunder in confusing symptoms 

 with maladies. In this sense, wheat rust is not a 

 disease : it is a symptom which betrays the presence 

 of a disease-inducing fungus, the Rust fungus. 

 Similarly, chlorosis is not a disease : it is a symp- 

 tom of imperfect chlorophyll action, and the best 

 proof of the truth of both statements is that in 

 both cases the fundamental disease-action is the 

 starvation of the cell-protoplasm of carbohydrates 

 and other essential food matters in the one case 

 because the fungus steals the carbohydrates as 

 fast as the leaves can make them, in the second 

 because the leaf is unable to make them. 



The foundation of a knowledge of disease in 

 plants therefore centres in the understanding of 

 the pathology of living cells. 



If a suitable mass of living cells is neatly cut 

 with a sharp razor the first perceptible change is 

 one of colour : the white " flesh " of a potato or 

 an apple, for instance, turns brown as the air 

 enters the cut cells, and the microscope shows that 

 this browning affects cell-walls and contents alike. 

 The cut cells also die forthwith ; and the oxygen 

 of the air combining with some of their constituents 

 forms the brown colouring matter which soaks 

 into the cell-walls. The uninjured cells below them 



