48 
The Mechanical Condition of the Soil 
after this process of growth has been excited, if it become 
checked, it cannot be renewed. This shows the necessity of 
keeping seeds dry when they are not required to germinate. 
Moisture alone is not sufficient for this process of growth, as the 
seed requires a supply of atmospheric air to enable the necessary 
chemical changes to proceed. Stagnant water in the soil must 
of necessity be unfavourable to germination, because it renders 
the land cold and excludes the free access of air, both of which 
conditions are prejudicial. 
The exceptions to this rule are very few : one, however, may 
be found amongst agricultural seeds in the floating sweet water- 
grass {Ghjccria fluitans), grown in our watei-meadows, in which 
instance immersion in water is absolutely necessary for the 
growth of the seed. In this case we have a seed which has 
the power of extracting its supply of air from water, — a power 
which but very few other seeds possess. The supply of air 
is as necessary for these aquatic seeds as for any others ; for 
if we drive out the air from water by boiling, they can no 
longer germinate. For the same reason, seeds which are buried 
deeply in the earth remain there for many years, not because they 
want moisture, but because it is unaccompanied by the presence 
of atmospheric air. The earth raised from wells, or brought 
from railway cuttings, or ploughed up by a furrow of extra depth, 
often becomes covered by a growth of vegetation, the produce of 
seeds which have long been dormant in the soil. 
Warmth is another essential condition for germination, which, 
within moderate limits, is rendered more rapid by an increase of 
temperature ; but it must be accompanied by a proportionate 
increase of moisture, otherwise it becomes destructive. The 
action of heat promotes chemical changes in the seed, but a 
free supply of water is necessary, not only that it may exert a like 
chemical influence, but also because it enters largely into the 
more delicate body into which the dry matter of the seed has to 
be transformed. Thus we see that healthy germination depends 
upon the combined action of the three agents — heat, water, 
and air. 
The opinions which are entertained respecting the influence of 
light are conflicting. Some consider that light retards the pro- 
cess of/'germination, whilst others consider that it does not influ- 
ence it prejudicially. The experiments which have been made, 
although far from conclusive, are calculated to favour the former 
opinion ; for the growth, although equally perfect, has not been 
as rapid under the action of light as when the seed has been 
covered from it. We know that, as soon as the seed has made 
sufficient growth to throw out its leaves, the action of light is 
favourable, its presence enabling the plant to decompose carbonic 
