BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.13. Biological Drawings

Purpose

1. To document experimental results for future reference.

2. To Complement visual observation and enable a more thorough and accurate examination of the specimen.

3. To aid memory and retention by sketching what you observe.

Rules

1. Use drawing paper of appropriate thickness and quality, which allows pencil lines to be easily erased.

2. Pencils must be sharp, HB grade, and non-colored.

3. The drawing must be:

a) sufficiently large — the more components the specimen contains, the larger the drawing should be; as a rule, it should occupy more than half of the page;

b) carefully executed — accurate proportions between different PARTS OF THE specimen must be maintained. If the specimen has multiple similar parts, their fine details must be rendered precisely;

c) drawn with fine, distinct lines — each line should be planned in advance and drawn in a single continuous stroke without lifting the pencil; do not use shading or coloring;

d) labeled as comprehensively as possible, with leader lines that do not intersect; leave enough space around the drawing for labels (preferably arranged vertically). Notes may be added regarding, for example, color intensity or shade, tissue shrinkage, or any unusual or, conversely, Characteristic Features of the sample.

4. When necessary, two drawings should be made: a) a schematic drawing showing the main features, and b) a detailed drawing of fine structures. For example, under low magnification, draw a plan of a plant cross-section showing only the different Tissues rather than individual Cells, and under high magnification, show the detailed cellular Structure (the enlarged section of the drawing should be outlined on the main plan using a bracket or a square).

5. Draw only what you actually see, rather than what you think you ought to see, and certainly do not copy illustrations from a textbook.

6. Each drawing must include a title, magnification, specimen orientation or projection (e.g., TS, LS, etc.), and explanatory notes. All of this should be placed in a designated Location, such as the upper right-hand corner of the page. Where possible, a scale bar should also be provided.

7. Leader lines pointing to labels (which should be drawn using a ruler) must terminate precisely at the structure being labeled, and all structures of potential interest should be labeled.

Experiment 5.1. Staining Starch in Plant Tissues

A dilute solution of iodine in potassium iodide (I₂/KI) can be used to stain starch in tissues a dark blue color. Starch is a carbohydrate typically stored in plant cells as small granules. Starch grains are easily observed by drawing a freshly cut potato tuber across a Glass Microscope slide and then staining the smear with the I₂/KI solution. In addition to starch, the I₂/KI solution stains lignified tissues (such as xylem and sclerenchyma) a bright yellow and unlignified tissues a pale yellow. Cell nuclei appear more distinct than the surrounding Cytoplasm.

Materials and Equipment

Microscope and light source

Eyepiece graticule and stage micrometer

Clean microscope slides and coverslips

Fine brush for section transfer

Lens cleaning tissue

Plain paper

Mounted needle

Stain dropping pipette (unless the stain bottle is supplied with a teat)

Filter paper

Iodine-potassium iodide solution

5% glycerol solution in a dropping bottle

A section of a plant organ, e.g., the stem of white deadnettle (Lamium album)

Preparation of I₂/KI solution

Dissolve about 1 g of iodine and 2 g of potassium iodide in 300 ml of Water. If a more concentrated solution is required, less water may be used. (Take normal safety precautions when handling iodine.) Thin fresh sections can be prepared using a razor blade by clamping the stem between two pieces of elder pith. Sectioning should be carried out in 70% alcohol.

Procedure

1. Using a brush, transfer the section selected for study onto a microscope slide and place it in the center of the slide.

2. Add two drops of I2/KI solution and one drop of 5% glycerol (to slow down evaporation and prevent the specimen from drying out too quickly).

3. Cover the section with a coverslip as shown in Fig. 5.39.

4. Examine the section carefully under the microscope. Pay particular attention to the distribution of dark-stained starch grains. Air bubbles with dark outlines, which are clearly empty, may appear in the preparation; these should be ignored.

5. Make a low-power drawing of the outline of the white deadnettle stem cross-section. The main objective is to accurately outline the area occupied by each tissue type. Keep in mind that:

a) in white deadnettle, a specialized supporting tissue—collenchyma—is present in each corner of the ridged stem.

b) vascular bundles vary in size. Additional strands of xylem and phloem sometimes develop between the vascular bundles, meaning this entire vascular tissue may eventually form a single continuous cylinder.

6. Label the low-power drawing and indicate the linear magnification of the drawing.

7. Identify which tissues contain starch. Starch grains are particularly abundant in the so-called starch sheath (endodermis). This should be drawn and labeled, and below the drawing, in a note, describe how the starch grains are distributed within its cells.

8. Test the statement that the average size of parenchyma cells in the stem cortex of white deadnettle is larger in the inner layers than in the outer layers.

To do this, take relevant measurements using an eyepiece graticule in both the inner and outer Regions of the cortex. Describe your method in detail. Record the results in eyepiece graticule units. Convert these measurements into micrometers by calibrating your eyepiece graticule.

Did your measurements support the statement in question?