Fig. 3.6-2a and b. Transverse section through an acorn (a seed of oak, Quercus). An easy way to identify starch grains is to illuminate them with polarized light. Fig. 3.6-2a is illuminated with ordinary brightfield (not polarized) light and the starch grains have been stained a pink color. This is typical of starch, but other storage products might also stain like this, so we could not be absolutely positive it is starch based just on this. Fig. 3.6-2b here is the same area illuminated with polarized light, and the crystalline nature of the starch grains is revealed by their being white with a dark cross running through each one. Notice that the walls these are primary walls because this is parenchyma do not shine; if these were secondary walls, they would shine as brightly as the starch grains (see Fig. 5.3-3).