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446 MODERN TIMES.
scopical formation was also closely examined, and the
discovery that the cell is the unique ultimate element of
their structure gave a new direction to morphological
investigation : it demanded that a greater attention should
be given to histogenesis. This was studied first upon the
lower cryptogams, since the facts to be dealt with in them
were simpler and less difficult to examine; then gradually
the more highly organized plants were taken in hand. The
results thus arrived at cast a remarkable light upon the
origin and growth of organs. MOHL observed various
modes of spore-formation, and in 1835 described an instance
of vegetable cell-partition. SCHLEIDEN in 1838 brought.
forward a theory of cell-formation, which however was disfigured
by so many mistakes that it was soon afterwards
abandoned. Its place was taken by the theory of NAEGELI
which was of a more comprehensive character, gave a
defined image of the different kinds of occurrences and
determined the law which controlled them. In 1839
SCHWANN enunciated the proposition that the animal cell
is analogous to the vegetable cell, and in 1855 UNGER
drew attention to the similarity between the protoplasm of
the vegetable cell and the sarcode of the most lowly
organized animals: this observation received increased
support by investigations made upon the myxomycetes.
These things led to a more correct appreciation of the
puzzling relations between the vegetable and animal king^
doms and also helped to remove the doctrine of the fixity
of species which had for long prevailed as an incontrovertible
dogma. The fecundation of plants was studied by
Du HAMEL who described the way pollination is effected,
and the part which is here played by many insects. This
subject was thoroughly worked at from 1830 onwards, the
processes which take place in the interior of the ovules
being made the subject of careful examination, and sexuality
being proved to exist even in cryptogams.
So, too, the processes of nutrition, of absorption, and
excretion, and of growth were fully described. The move-