THEIR NATURE AND HABITS. 
199 
two valves placed together in a position somewhat similar to a 
closed bivalve shell. Within this shell is found a yellowish-brown 
or occasionally green colouring matter or endochrome, which is 
similar in composition to the chlorophyll of most plants and 
evidently capable of performing similar functions. Diatomine 
was the term employed by Hageli to indicate the colouring matter 
of diatoms, which he considered to be a simple compound. The 
absorption-spectra produced by chlorophyll are also obtained by 
diatomine, indicating at least a close similarity in their con¬ 
stitution. The colour of diatomine is readily changed to an emerald- 
green by the application of heat, alcohol, or acid. On adding chloro¬ 
form to a diluted alcoholic solution, Petit found that on separation 
the whole of the yellow colouring matter was left in the alcohol 
while the chloroform was coloured green by a substance probably 
identical with ordinary chlorophyll. Some extraordinary instances 
have been given relative to the apparent power of this endochrome 
to resist decomposition, a character which, I believe, is entirely 
absent in the usual colouring matter of plants. Bran relates that 
some diatoms which he had collected in the Sahara and kept in 
the water in which they were found, preserved their endochrome in 
good condition for four years. The same authority also relates that 
he had seen fossil diatoms from a large deposit in Holland, and 
which must have been buried for centuries, present occasional 
specimens in which the endochrome was still yellow and trans¬ 
parent, although it had become somewhat thicker and more plastic. 
He states his conviction that this only applies to specimens which 
had reached perfect maturity, and whose valves were still tightly 
closed. Although, however, when found, the frustule may be 
hermetically sealed, thus preserving the contents from exterior 
agencies, it is difficult to understand why, during the early stages 
of fossilization, foreign matter should not enter through the 
apertures which exist at the extremities of all frustules. It would 
be well to receive still further confirmation before regarding it 
as conclusive that undecomposed endochrome actually exists under 
these conditions. 
Protoplasm is also necessarily present in the frustule, but remains 
invisible until coloured by some special re-agent. It is then seen 
attached to the shell and occupying a position similar to that of the 
primordial utricle in most vitally active cells. 
Unlike the great majority of plants, diatoms are believed to 
contain no starch. It is possible that traces may yet be discovered, 
but if present at all the quantity must be exceedingly small. 
Starch being now considered to be one of the first-formed products 
of assimilation when chlorophyll decomposes the carbon-dioxide of 
the atmosphere, it would be naturally expected to be similarly 
formed under the action of diatomine. In the centre of the fluid 
contents of the frustule is frequently seen a small transparent 
globular body, which from its position and character has been 
termed the nucleus ; it is doubtful whether this has any connection 
with the nucleus found in the protoplasm of young cells. There 
