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PART III. PHYSIOLOGY. 



[45 



tion. The inorganic residue is a white, or, if the combustion is 

 imperfect, a grey powder, the ash. 



As the result of chemical processes attending the .combustion, 

 the sulphur and phosphorus previously contained in the organic 

 compounds appear as sulphates and phosphates in the ash, and the 

 carbonic acid formed during combustion combines with some of 

 the inorganic substances. These, therefore, must not be included 

 in an accurate estimate of the constituents of the ash. 



The ash usually constitutes but a small percentage of the whole 

 dry solid of the plant. The amount of ash increases with the age 

 of the plant, or of any part of it, inasmuch as there is no consider- 

 able excretion by the plant of the mineral substances absorbed. 

 The percentage of ash in the dry solid of the plant, or of any 

 organ, may vary widely at different times. The following analyses 

 of various portions of plants will give an idea of its amount and 

 composition : 



1000 PAKTS OF DEY SOLID MATTER CONTAIN : 



2. The Food of Plants. The constituents of the ash do not 

 form a merely accidental mixture ; it has been proved by experi- 

 ment that certain inorganic compounds are absolutely necessary to 

 the life of the plant. Those chemical elements which the plant 

 requires for its nutrition, and which must therefore be regarded as 

 part of its food, are : 



I. Non- metallic Elements : Carbon, hydrogen, oxygen, nitro- 

 gen, sulphur, phosphorus, and perhaps chlorine. These 

 elements exist in the plant, for the most part, as organic 

 compounds ; but they also occur to some extent as 

 inorganic compounds, carbonates, nitrates, phosphates, 

 sulphates, of the metals mentioned below. 

 II. Metallic Elements : Potassium, calcium, magnesium, iron. 



