With the light thus cast upon the question, we may conclude that the
sensitive period commences not only at the beginning of the germination,
but must also be considered to include the life of the seed itself. From
the moment of fertilization and the formation of the young embryo the
development must be subjected to the influence of external agencies
which determine the direction it will take and the degree of development
it will finally be able to acquire. Probably the time of growth of the
embryo and of the ripening of the seed correspond exactly to the period
of highest sensibility. This period is only interrupted during the
resting stage of the seed, to be repeated in germination. Afterwards the
sensibility [389] slowly and gradually decreases, to end with the
definite decision of all further growth sometime before the outer form
of the organ becomes visible under the microscope. The last period of
life includes only an expansion of the tissues, which may still have
some influence on their final size, but not on their form. This has been
definitely arrested before the end of the sensitive period, and
ordinarily before the commencement of that rapid development, which is
usually designated by the name of growth, as contrasted with evolution.
Within the seed the evolution of the young plant manifestly depends upon
the qualities and life-conditions of the parent-plant.
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