indeed, but showing
no uniform and elemental structure. It was Schwann's merit to interpose
between the tissue and the mere unorganised material a new element of
structure, the cell. And, as it happened, a few years before Schwann
published his cell-theory, Dujardin hinted at another degree of
composition which was later to take its place between the cell and the
chemical elements--sarcode or protoplasm.

As is well known, the concept of the cell arose first in botany. Robert
Hooke discovered cells in cork and pith in 1667, and his discovery was
followed up by Grew and Malpighi in 1671, and by Leeuenhoek in 1695. But
they did not conceive the cell as a living, independent, structural
unit. They were interested in the physiology of the plant as a whole,
how it lived and nourished itself, and they studied cells and
sieve-tubes, wood fibres and tracheae with a view rather to finding out
their functions and their significance for the life of the plant than to
discovering the minutiae of their structure. The same attitude was taken
up by the few botanists who in the 18th century paid any heed to the
microscopical anatomy of plants. For C. F. Wolff,[238] the formation of
cells was a result of the secretion of drops of sap in the fundamental
substance of the plant, this substance remaining as cell-walls when
cell-formation was completed--no idea here of cells as units of
structure.

In the early 19th century, interest in plant anatomy revived somewhat,
and much work was done by Treviranus, Mirbel, Moldenhawer, Meyen and von
Mohl.[239] As a result of their work the fact was established that the
tissues of plants are composed of elements which can, with few
exceptions, be reduced to one simple fundamental form--the spherical
closed cell. Thus the vessels of plants are formed by coalescence of
cells, fibres by the elongation of cells and the thickening and
toughening of their walls. At this time, interest was concentrated on
the cell-wall, to the almost total neglect of the cell-contents; the
"matured framework" of plant cells, to use Sach's convenient phrase, was
the chief, almost the sole, object of study. And it was natural enough
that the mere architecture of the plant should monopolise interest, that
the composition of the tissues out of the cells, and the fitting
together of the tissues to form the plant should awaken and hold the
curiosity of the investigator; even the modifications of the cell-walls
themselves, their r