Application of Polarizing Microscopes in Starch Observation
Polarizing microscopes are widely used in mineralogy, chemistry, biology and botany. For example, in botany, they are used to identify fibers, chromosomes, spindle filaments, starch grains, cell walls, and to detect crystals in cytoplasm and tissues. In medicine, polarizing microscopes are adopted to examine crystals in synovial fluid. We know that starch can be identified by iodine staining. However, a polarizing microscope can identify starch without staining. The following describes potato starch grains: cut a small piece of potato, smear it on a glass slide, add a drop of water to prepare a wet mount, and then observe it. Under low magnification, the unstained particles look ordinary.
Ordinary starch is white or off‑white. It is insoluble in organic solvents such as ether, ethanol and acetone, and also insoluble in cold water. Starch exists as granules in endosperm cells. Starches from different sources vary in shape and size. Microscopic observation can distinguish different types of starch or identify unknown samples. Starch granules are generally divided into three shapes: round, oval and polygonal. Plant starch granules with high water content and low protein content are usually larger, mostly round or oval, such as potato starch. On the contrary, granules with low water and high protein content are smaller and polygonal, such as rice starch. When observed at 400–600× magnification, concentric striations similar to tree growth rings can be seen on the surface of some starch granules; the striations of potato starch are particularly obvious.
When the polarizer is rotated, a distinct cross pattern appears on the starch grain. This cross is known as the Maltese cross, and its intersection is located at the hilum of the starch grain. The Maltese cross has its own research history. At higher magnification, the concentric rings of the starch grain can be observed, with the central point marking the position of the hilum.
