84 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 can, 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, pho-
phorus, sulphur, calcium, magnesium and iron. Knop's
Solution consists of 2 grm. of calcium nitrate, and .5
grm. 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
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;