Page 59 - Algae

42 Algae: Anatomy, Biochemistry, and Biotechnology

FIGURE 2.9 Body scale of Mallomonas crassisquama.

frustule. They exist in the form of both thick mucilaginous capsules and thin tightly bound organic
sheaths. The formation of the frustule has place in the silica deposition vesicles, derived from the
Golgi apparatus, wherein the silica is deposited. The vesicles eventually secrete their finished
product onto the cell surface in a precise position.
Diatoms can be divided artificially in centric and pennate because of the symmetry of their frus-
tule. In centric diatoms, the symmetry is radial, that is, the structure of the valve is arranged in refer-
ence to a central point (Figure 2.10). However, within the centric series, there are also oval,
triradiate, quadrate, and pentagonal variation of this symmetry, with a valve arranged in reference
to two, three, or more points. Pennate diatoms are bilaterally symmetrical about two axes, apical
and trans-apical, or only in one axis, (Figure 2.11); some genera possess rotational symmetry,
(cf. Chapter 1, Figure 1.30). Valves of some pennate diatoms are characterized by an elongated
fissure, the raphe, which can be placed centrally, or run along one of the edges. At each end of
the raphe and at its center there are thickenings called polar and central nodules. Addiction
details in the morphology of the frustule are the stria, lines composed of areolae, and pores
through the valve that can go straight through the structure, or can be constricted at one side.
Striae can be separated by thickened areas called costae. Areolae are passageways for the gases,
nutrients exchanges, and mucilage secretion for movement and attachment to substrates or other
cells of colony. Other pores, also known as portules, are present on the surface of the valve.