Diatoms Desmids Light Microscope

Written by microscopes

Saturday, 04 August 2007

A description and examination of Diatoms and Desmids in the Light Microscope.

Diatoms and Desmids in the Light Microscope

The desmids and the diatoms are two closely related groups of aquatic plants. Some difficulty will be experienced in distinguishing one from the other, but after a little study the differences will become apparent.

The desmids and the diatoms are two closely related groups of aquatic plants. Some difficulty will be experi- enced in distinguishing one from the other, but after a little study the differences will become apparent.

Desmids are usually found in the sweetest and freshest water. Salty or brackish water contains none at all, while diatoms flourish there as well as in a mill-pond. Living desmids are always green, diatoms are always brown. There are other means of identification, such as flatten- ing the soft cell wall of a desmid by pressing the cover glass down against the slide, and rupturing the cell wall. The green coloring matter (chlorophyl) and the color- less protoplasm that fills the cell may then be forced out. The cell wall of a diatom is hard and brittle, being com- posed of silicate. The cover glass may be pressed down on a diatom until the glass breaks without flattening or changing its shape. It may roll over and change its posi- tion, presenting an aspect quite different from that first seen, but it will probably roll back again and appear as it did at first. If the cell wall is fractured the break will not be irregular or of the appearance presented by a soft wall when broken, but will display the characteristic fracture of a hard, glass-like substance.

Both desmids and diatoms have the power of locomo- tion, frequently moving from place to place. When mixed with mud, as they probably will be when collected with a net, desmids slowly work themselves free and rise to the surface where they collect in a green scum or line at the side of the vessel nearest the window, whence they may be taken for examination. Diatoms have a similar power of motion, but they usually move more rapidly. Under the microscope the desmids may be seen moving slowly and sedately across the field in a straight line. The diatoms start across the field, get half way, then stop and retreat or go in an entirely different direction. They always seem to have important business to do and to be in a tremendous hurry to get it done. Thus an object that may seem to be either a diatom or a desmid is not a diatom if it moves slowly, nor is it a desmid if it darts around the field like a humming bird.