. The change thus effected takes place also in a precisely
similar manner in the plant, diastase being produced during the process
of germination of all seeds and tubers, for the purpose of effecting
this change, and to fulfil other functions less understood, but no doubt
equally important. Diastase is found in the seeds only during the period
when the starch they contain is passing into sugar; as soon as that
change has taken place, its function is ended, and it disappears.

CHAPTER III.

THE CHANGES WHICH TAKE PLACE IN THE FOOD OF PLANTS DURING THEIR GROWTH.

The simple compounds which the plant absorbs from the atmosphere and
soil are elaborated within its system, and converted into the various
complex substances of which its tissues are composed, by a series of
changes, the details of which are still in some respects imperfectly
known, although their general nature is sufficiently well understood.
They may be best rendered intelligible by reference, in the first
instance, to the changes occurring during germination, when the young
plant is nourished by a supply of food stored up in the seed, in
sufficient quantity to maintain its existence until the organs by which
it is afterwards to draw its nutriment from the air and soil are
sufficiently developed to serve that purpose.

_Changes occurring during Germination._--When a seed is placed in the
soil under favourable circumstances, it becomes the seat of an important
and remarkable series of chemical changes, which result in the
production of the young plant. Experiment and observation have shown
that heat, moisture, and air, are necessary to the production of these
changes, and though probably not absolutely essential, the absence of
light is favourable in the early stages. The temperature required for
germination varies greatly in different seeds, some germinating readily
at a few degrees above the freezing point, and others requiring a
tolerably high temperature. The rapidity with which it takes place
appears to increase with the temperature; but this is true only within
very narrow limits, for beyond a certain point heat is injurious, and
when it exceeds 120 deg. or 130 deg. Fahrenheit, entirely prevents the process.
The presence of oxygen is also essential, for it has been shown that if
seeds are placed in a soil exposed to an atmosphere deprived of that
element, or if they be buried so deep that the air does not reach them,
they may lie without change