THE NUTRITION OF THE EOOT 33 



whieh the last two elements occur in the same proportions as 

 they do in water. 



'Starch, sugar, and cellulose are such carbo-hydrates which 

 are transmutable within the plant. If a plant does not find 

 any potassium in the soil, its growth ceases, and the leaves do 

 not develop the power of forming starch within the green 

 colouring bodies, the chlorophyll grains. A plant which has 

 perhaps shown no growth for months, owing to the absence of 

 potassium, will recover in a few days after that element has 

 been supplied to it ; even after a very few hours starch will 

 begin to imake its appearance in the chlorophyll grains. The 

 very, soluble salts of this important nutritive substance are 

 liable to become washed out of the soil if watering is too 

 continuous. 



Calcium, the foundation of all combinations, of lime, is 

 chiefly concerned in the strengthening of the cell-wall, within 

 which it is often deposited in visible crystals of carbonate of 

 lime. But besides this, it is important in fixing the oxalic 

 acid which is produced by plants, and which is poisonous to 

 them. In this process of fixing, crystals of calcium oxalate 

 are formed, which in many plants are stored in continuous 

 rows in the cells of the cortex immediately surrounding the 

 channels through which the plastic material passes, and show- 

 ing therefore their intimate connection with the metabolic 

 processes. For as the organic substances passing downward 

 from the leaves through the cortex undergo manifold changes, 

 chiefly of the nature of further oxidation, it is probable 

 that the last function of the lime is to fix the oxalic acid, 

 which is formed by the oxidation of the carbo - hydrates, 

 and thus to render it harmless. Calcium is taken up by the 

 plant in the form of sulphates, phosphates, and carbonates. 

 In the case of water-cultures, calcium nitrate may be advan- 

 tageously used. 



Magnesium, which of all the nutritive substances is most 

 nearly allied to calcium, but cannot replace it in the vegetable 

 economy, seems to have quite a different effect on the plant. 

 It seems to work together with the nitrogen in the formation 

 of protoplasm, and to influence the formation of chlorophyll ; 

 for plants cultivated without magnesium only form yellowish- 



c 



