Calibrating the microscope: You cannot tell, in absolute terms, how big any specimen under your microscope is without calibrating the microscope first. You will always have an ocular micrometer in one of your oculars, but using this alone gives only a relative indication of size. You must use a stage micrometer to calibrate the scope, and in doing so, you will find out:

What does one ocular unit represent in absolute units?

You can then use the ratio you found to calculate the size of specimens. You must calibrate the scope at each magnification that you intend to use.

Something about calibration, or its downstream activities (calculating size, creating a scale bar) is always on the lab exam.

Plant tissues: We looked only at a vascular bundle from the stalk of Swiss Chard (Beta vulgaris). The vascular bundle is made up of a cap of smaller tubes known as phloem. These tubes carry the plant's food (what is the plant's food, where does it come from?). The larger, thicker-walled tubes are the xylem vessels; these transport water. The parenchyma cells are the larger, blockier cells which surround and support the vascular bundles.

Cross Section			Try as I might, I couldn't get a decent picture of a vascular bundle, so I've provided links to two shots. In the first picture, the massive squarish patch of large white cells is made up of parenchyma. The xylem borders this (stained red), the phloem is the beige (unstained), crescent-shaped patch adjacent to the xylem. This photo is not from celery or Swiss chard, thought the bundles are representative. In the second picture (from celery), the parenchyma is the mass of purple-staining cells surrounding the bundle. Xylem tubes are large and thick-walled (the cluster on the left), the phloem are smaller and form into an arc-shaped bundle.
Longitudinal Section			So I was a bit better at the longitudinal sections. In the first slide, the red bracket denotes one vascular bundle. The second slide is at higher mag, and shows the curly nature of the xylem wall.

Animal tissues: The study of tissues is known as histology (this term applies to animals and plants). There are four basic tissue types in animals, we looked only at epithelial tissues. Epithelium lines cavities or covers an organ; ergo, look for these tissues next to some type of space. Recall that the word simple, when used in a histological sense, means one layer; in contrast, stratified refers to an epithelium of more than one layer of cells.

Specimen	broad view	zoomed in	notes
simple squamous epithelium - lung			Squamous - sounds like squished. Good reason for that. In the first slide, the ellipse shows one alveolus. Note in the second slide how a squamous cell bulges in the position held by the nucleus.
simple cuboidal epithelium - kidney			This is from one of the urine-producing tubes known as a nephron.
simple columnar epithelium - small intestine			Look for goblet (*) and absorptive cells. In the latter, note that the nuclei are more or less all at one level. The open brackets denote the single layer (simple) of cells making up this tissue. The arrow points to the brush border.
pseudostratified ciliated columnar epithelium - trachea			What is it about this simple tissue that gives it a stratified appearance?
fundic area of the stomach			Look for both parietal and chief cells.

 

R.F. Lauff Department of Biology St. Francis Xavier University Antigonish, NS Canada B2G 2W5