In this article we will discuss about the thallus structure of Navicula with the help of suitable diagrams.
Thallus is represented by a iso-bilaterally symmetrical diploid unicells.
Structurally, it can be differentiated into two parts – A siliceous cell wall (called frustule) and the protoplast. The cell wall is made of pectic substances which are impregnated with silica (SiO2). Cell wall consists of two overlapping halves called epitheca and hypotheca. Epitheca remains fitted over the hypotheca as a lid over the box.
Each theca is further divided into two parts:
The main surface called valve and the incurved margin known as connecting band or cingulum. The two connecting bands of the two thecas are fitted together. The connecting band of the epitheca overlaps that of the hypotheca and the two bands remain united in the overlapping region (called girdle) by a connecting cement present between the (Fig. 1).
A frustule can be seen in two views:
Valve view and girdle view. In top view or valve view it appears as boat shaped (Fig. 2A) and in girdle view or side view more or less rectangular (Fig. 2B). The valve view shows marking or striations which spread out laterally in two parallel series, one on either side of the axial strip. The axial strip pears a longitudinal cleft known as raphe (Fig. 2B).
The raphe extends from one end of the valve to the other. It also bears three enlargements or rounded nodules, one central nodule and two polar nodules. Due to presence of raphe Navicula shows gliding movement. This movement is caused “by streaming cytoplasm by circulation within the raphe, and by the extrusion of the mucilage.”
Just inner to the cell wall is present a plasma membrane which encloses the cell protoplast It is differentiated into a single nucleus and cytoplasm. The cytoplasm forms a thick layer just below the cell wall and encloses a large central vacuole.
The cytoplasm includes mitochondria, Golgi bodies, and two large parietal brownish yellow chromatophores. Pyrenoids are absent. The photosynthetic pigments are chlorophyll-a, Chlorophyll-c, p-carotene, fucoxanths, diatoxanthin and diadinoxanthin. Reserve food material is in the form of chrysolaminarin and oil droplets.