e influence exerted by manures on
the composition of crops. It has been assumed in the preceding pages
that the composition of crops of the same plant is uniform; but this is
not strictly the case, as it has been proved that not merely the manure
and soil have an appreciable influence on the crop's composition, but so
also has the climate.

_Absorption of Plant-food._

The laws regulating the absorption of plant-food are most interesting,
although, unfortunately, very imperfectly understood as yet. The
fertilising ingredients are capable of considerable movement in the
plant, and are only absorbed up to a certain period of growth. This in
many plants is reached when they flower. After this period they are no
longer capable of absorbing any more food. The popular belief that
plants in ripening exhaust the soil of its fertilising matters is
consequently a fallacy.

_Fertilising Ingredients lodge in the Seed._

The tendency of fertilising matters is to move upward in the plant as it
matures, and finally to become lodged in the seed. It is for this reason
that the cereals prove such an exhaustive crop. That nature, however,
can in certain cases be very economical of her food-supplies, is
strikingly illustrated by the fact that much of the fertilising matter
contained in the mature leaves in autumn passes back into the tree
before the leaves fall from it.

_Forms in which Nitrogen exists in Plants._

The form in which nitrogen is present in the plant is chiefly as
albuminoids. As, however, albuminoids belong to that class of bodies
known as colloids, which cannot easily pass through porous membranes
like those forming the walls of plant-cells, they are changed during
certain periods of the plant's growth into amides, which are
crystalloids, and consequently able to move freely about in the plant.
Amides are most abundant in young plants during the period of their most
active growth, and as the plant ripens the amides seem to be largely
converted into albuminoids.

While the subject is not very clearly understood, it would seem to be
pretty conclusively proved that there is a direct relation between the
amount of the phosphoric acid and of the nitrogen absorbed.

_Bearing of above Facts on Agricultural Practice._

The bearing of these facts upon practice is obvious. In the first place,
they show how important it is that plants should be well fed when they
are young, and that in the practice of green manurin