chitinous spines than they do a form without them, presumably because longer spines
make the cell more readily detected and captured more effectively. The whole matter
of spines is still not fully explained.
Fig. 2.8 Phase-contrast light micrograph of Chaetoceros decipiens, showing chaining
of cells and fine siliceous spines (spines this fine are usually called setae).
(^) (Courtesy of J.D. Pickett-Heaps, University of Melbourne.)
(^) Diatom cells divide by an unusually elaborate process, since the cell not only must
divide, but must fabricate new valves. Pickett-Heaps et al. (1990) have reviewed
much of what is known about diatom mitosis. Division (Fig. 2.9) proceeds inside the
old frustule until there are two protoplasts, one adjacent to each valve. Each has its
own cell membrane at the central plane. New siliceous walls form inside and parallel
to each of these membranes. Formation begins with the appearance of membranous
vesicles, probably derived from one or more sets of Golgi apparatus (cell organelles
involved in packaging of secretory products, particularly protein–carbohydrate
complexes) in each cell. These vesicles aggregate in the center of the division plane,
and coalesce to form a “silicalemma”, a membrane surrounding a silica deposition
vesicle (SDV). The two sides of the silicalemma may connect in a “donut hole”
wherever there is to be a pore in the new frustule. There are other elaborations
equivalent to at least some of the sculpture of the new frustule, although it is not
certain that all details are sculpted in that manner. The vesicle fills with silica with
remarkable rapidity, in minutes.
Fig. 2.9 Sequence of cell division and frustule formation events in the pennate diatom
Gomphonema parvulum. (a) Cell elongates, additional volume covered by addition of