Family III, — Diatomacece. (Plate XIV.)

Like the former family, the Diatomacece are unicellular plants,
in some instances isolated, in others cohering in chains of fila-
ments, or in some definite way. The cell wall, however, is
composed of a glassy or silicious material, instead of cellulose,
which is found in all other vegetable cells ; and the endo-
chrome is usually of a rich amber tint instead of green.
They exhibit also much symmetry and beauty in the forms of
the frustules, which are often so exquisitely sculptured as to
afiford excellent test objects for the microscope.
Each frustule consists of a new and an old half or valve, as
noticed in the Desmidiacea^, but the margins of the old valve
overlap those of the new one, and thus results the so-called
cingulum or " middle piece,** which is not only capable of
elongation by growth, but also by one portion sliding upon
the other, telescope fashion, so as to make provision for the
endogenous development of two new half frustules by fission
and gemmation combined. From this arrangement it follows
that the cells of each successive generation must be narrower
than those within which they arise, by at least the whole thick-
ness of the cell wall. Here, then, is the explanation of the
great disparity of size so frequently observed in members of
the same species. * Moreover, we thus also see why it is that
after the conjugation of two frustules, the resulting sporangieil
cell, in which the process just described commences, should be
so much larger than the parent cell.
The genera of Diatomacece are too numerous to be sepa-
rately defined in this treatise ; but the annexed table, with the



42



CLASSIFICATION OF DIATOMACILE.
