272 DISEASE IN PLANTS. 



health, and which when interfered with result in 

 disease and death. The protoplasm of the living 

 plant-cell looks like a slimy translucent mass 

 which has been superficially compared in appear- 

 ance to well-boiled sago or clear gum. Fifty 

 years of observations and experiments with it 

 have convinced physiologists that it is not a mere 

 solution or emulsion, however, or even a chemical 

 compound in the ordinary sense of the term, 

 although chemical analysis gets little out of it 

 beyond water, proteids, carbohydrates and fats, 

 and traces of certain mineral salts ; for living 

 protoplasm does not respond to the laws of 

 physics and mechanics in obeying them, simply 

 as do ordinary solutions and liquids. On the 

 other hand, the most delicate chemical manipu- 

 lation fails us, because when killed it is no 

 longer protoplasm. Nor does the microscope 

 advance matters far, beyond convincing us that 

 this marvellous material must have a structure 

 far more intimate than anything visible to the 

 highest magnifying powers at our disposal. 



Nevertheless, some information is forthcoming 

 from the comparative examination of the pro- 

 toplasm of numerous different kinds of organisms, 

 for we have learnt that certain ingredients and no 

 others are necessary for its composition namely, 

 carbon, hydrogen, oxygen, nitrogen, phosphorus, 

 sulphur, calcium,^ magnesium, potassium and it 

 is as a rule of no use trying to foist on to it any 

 substitute for any one of these. Moreover, these 



^ See note at end of chapter. 



