ones, if any, will appear in the boiled water, and they will soon
cease growing. Why is this? To answer this question, try another
experiment. Take two bottles, filled as before, one with hydrant water
and the other with boiled water; drop into each a slip of glass or a
spoon or piece of metal long enough so that one end will rest on the
bottom and the other against the side of the bottle, and let stand for
an hour or so (Fig. 17). At the end of that time bubbles of air will
be seen collecting on the glass or spoon in the hydrant water, but
none in the boiled water. This shows us that water contains more or
less air, and that boiling the water drives this air out. The cutting
in the boiled water did not produce roots because there was no air in
it and the oil kept it from absorbing any.

=Experiment.=--Into some tumblers of moist sand put cuttings of
several kinds of plants that root readily (Fig. 18), geranium,
tradescantia, begonia, etc. Put cuttings of same plants into tumblers
filled with clay that has been wet and stirred very thoroughly, until
it is about the consistency of cake batter. Keep the sand and puddled
clay moist; do not allow the clay to crack, which it will do if it
dries. The cuttings in the sand will strike root and grow, while most,
if not all, those in the clay will soon die. The reason for this is
that the sand is well ventilated and there is sufficient air for root
development, while the clay is very poorly ventilated, and there is
not sufficient air for root growth.

These experiments show us that to develop and do their work roots need
air or a well-ventilated soil.

We have found the conditions which are necessary for the growth and
development of plant roots, namely: