Q4 BOTANY 
nitrogen as well as the rest of their food from the host 
on which they live. 
Certain Bacteria, which live on the roots of the 
bean and other plants of that family, have the power 
of taking free nitrogen from the air and building it 
up into the substance of their bodies. | When these 
bacteria die, they form humus, which eventually pro- 
duces nitrates available for the plant. It is for this 
reason that plants like clover and lucerne ean, if 
properly treated, grow on soil poor in nitrogen. 
WATER CULTURES. 
By growing plants in distilled water to which 
certain mineral substances have been added, it is 
possible to discover the relative importance of the 
different elements that go to make a perfect plant 
food. Since the carbon of the plant comes from the 
air, and its oxygen and hydrogen from the water it 
only remains to supply its nitrogen, potassium, phos- 
phorus, sulphur, calcium, magnesium and iron. Knop’s 
Solution consists of 2 germ. of calcium nitrate, and °5 
erm. each of potassium nitrate, magnesium sulphate ; 
and potassium phosphate with a few drops of iron 
chloride, all dissolved in four or five litres of distilled 
water. 
Now take eight equal sized shoots of Wandering 
Jew. Place one in a solution to which all of the above 
substances have been added. To deprive one shoot of 
potassium use sodium nitrate instead of potassium 
nitrate and caleium phosphate instead of potassium 
phosphate. Deprive others of calcium by omitting 
calcium nitrate, of phosphorus by omitting the 
potassium phosphate, of magnesium by using calcium 
sulphate in place of magnesium sulphate; of sulphur, 
by using magnesium chloride instead of the sulphate; 
of nitrogen, by using potassium chloride and calcium 
sulphate in place of calcium and potassium nitrate: 
