DESCRIPTION OF PLATES. 189 
The preparation and mounting of mixed dia- 
toms in balsam is not so difficult a matter, and 
every microscopist should make himself familiar 
with the technique, which is given in the Micro- 
scopic Dictionary and in Dr. Carpenter’s admira- 
ble work, which has already been frequently 
referred to. 
The fascinating work of collecting and mount- 
ing cabinet specimens of the silicified valves 
commonly known as diatoms, should not occupy 
the attention of the amateur microscopist to the 
exclusion of the study of the life-history of the 
unicellular plants which they enclose. 
The usual mode of increase is the same as 
among other unicellular organisms : viz., by binary 
division. This occurs in a plane parallel with 
the flat surfaces of the two valves, by a division 
of the cell contents, and the formation of a double 
membranous partition, in each wall of which 
silex is deposited to form a new valve. T'wo indi- 
viduals are thus formed like the parent-form, 
which when separated have each one old and 
one new valve. 
Conjugation also occurs among the Diatomacee. 
We have already seen that this curious mode of 
reproduction is common among unicellular or- 
ganisms. In the case of diatoms, it occurs 
between two cells —/frustules — which, lying in 
proximity, open at the margin, — sutwre,— and 
permit of the extrusion of the cell-contents, 
which coalesce and form a new frustule. This 
becomes invested with a silicified casing, like that 
