182
THE RARE SPIRAL DIATOM CYLINDROTHECA
GRACILIS AND ITS OCCURRENCE IN
EPPING FOREST.
By D. J. SCOURFIELD, I.S.O., F.L.S., F.R.M.S.
(Read 28th November, 1942.)
THE little diatom,1 Cylindrotheca gracilis (Brebisson) to
which I wish to draw attention in this paper is remarkable
in several ways—in its structure, in its movement, in its flexi-
bility and probably also in its mode of division.
Before dealing with its special peculiarities, however, a few
words may be devoted to its general appearance. It is a very
elongated form as a rule, barely 1/5000" (5u) in greatest width,
but twenty to thirty or more times as long as broad (Fig. 1).
The main central part is nearly parallel sided, but tapers more
or less rapidly into very slender terminal parts each of which
expands somewhat at the end often forming a minute knob
which, however, is slightly broader in one view than at right
angles to it (Figs. 8 and 9). The slender prolongations of
the main body are usually in line with the principal axis, but
are sometimes slightly curved, giving a much elongated sigmoid
effect to the whole frustule. Internally there is a fairly evident
nucleus in the centre, and on each side of this a number of
chromatophores, usually three or four but sometimes more.
A few oil globules may also be present. Superficially it is not
very different from some other elongated diatoms, e.g. Nitzschia
gracilis, Nitzschiella acicularis and Synedra acus.
As regards the peculiar structure of Cylindrotheca2 perhaps
the feature most easily observed in the living state is that, which-
ever way it may be lying, it is always of the same width and shape.
This implies that it is circular in section. Now this is a unique
feature among the Pennales, the Order of diatoms to which it
is now known to belong3 (see Hustedt 1928 and 1930, Fritsch
1935), for in all other members of this Order the valve view and
the girdle view are different in shape as well as in detail.
1 For the benefit of those who are not familiar with these microscopic organisms it may
be mentioned that diatoms are unicellular aquatic plants forming one of the Classes of
the large group of lower plants known as Algae. Each cell, or frustule, is enclosed in a
siliceous case consisting of two valves which typically, each with a siliceous hoop attached,
the two hoops forming the so-called girdle, fit over one another like the two parts of a pill-box.
The characteristic colour of diatoms is not green, but deeper or lighter shades of golden brown
and is due to distinct chromatophores. The Class is divided into two Orders, the Centrales
and the Pennales which appear to be very distinctly marked off from one another. Many
forms among the Pennales are motile and these all possess a structure known as a raphe which
is a longer or shorter open canal on the valve face or on a keel arising from the valve.
2 As only one species of Cylindrotheca has been recognised there is no need to use more
than the generic name in the remainder of this paper.
3 Owing largely to its circular section it was regarded as one of the Centrales by most
authors for quite a long time, e.g., see West and Fritsch (1927).