(B) Food Reserves

Re-examine the slide (fresh mount) of Spirogyra. Do you notice the small dots embedded in the chloroplast? These are called pyrenoids and are the site of starch storage in the green algae. Starch will stain blue-black in the presence of iodine. Place a drop of IKI (iodine stain) to one side of your cover slip, then carefully touch the corner of a KimWipe tissue to the liquid on the opposite side. This will draw the stain through the slide mount. Watch the pyrenoids during this process. What color do they appear? Try this test with some of the other green algae available in lab. Where are the pyrenoids located?

Try the test with a red alga. The red algae have a storage product similar to starch, but the bonding in the glucose molecule chains is different. The color reaction is usually brown. Did you observe any pyrenoids?

Finally, observe the diatoms. Try the iodine stain. Diatoms store oil droplets as their primary food reserve. If these diatoms have been growing under optimal conditions, you should be able to observe the oil droplets dispersed throughout the cell.