Microscope calibration, cell drawings and plant tissues

What you’ll learn

• Calibrate an eyepiece graticule and calculate actual size or magnification.
• Distinguish a tissue plan from a cell drawing.
• Identify plant transport tissues using supplied structural evidence.

Understand the idea

Core practicals 5 and 7 connect animal-cell observations with plant-cell and tissue observations. An eyepiece graticule has arbitrary divisions until it is calibrated against a stage micrometer at the selected objective. Recalibrate when the objective changes. Magnification compares image size with actual object size using the same length units; resolution describes the ability to distinguish close points, so increasing magnification alone does not necessarily reveal more detail.

A biological drawing should record the structures actually visible, with clear boundaries, accurate proportions and unambiguous label lines. A plan diagram shows the arrangement and relative sizes of tissues rather than individual cells. A detailed cell drawing instead shows the observable cellular features. Include an appropriate scale bar or stated magnification, and do not draw organelles simply because a textbook says they exist if the chosen microscope cannot resolve them.

In plant sections, identify tissues from a combination of position and structure. Xylem vessels commonly have large lumens and thick lignified walls; phloem includes sieve-tube elements and associated companion cells; sclerenchyma fibres have thick supporting walls. Their arrangement differs among roots, stems and leaves, so a generic pattern should not replace the section's actual evidence. Stains improve contrast, while thin, well-prepared sections reduce overlapping structures; preparation is supervised where blades or stains are used.

Key formulae

$$\mathrm{size\ per\ graticule\ division}=\frac{\mathrm{known\ stage\ length}}{\mathrm{matching\ graticule\ divisions}}$$

$$M=\frac{\mathrm{image\ size}}{\mathrm{actual\ size}}$$

$$1\ \mathrm{mm}=1000\ \mu\mathrm{m}$$

Worked example

At one objective setting, 40 graticule divisions span $100\ \mu\mathrm{m}$. A cell spans 18 divisions and its drawing is $36.0\ \mathrm{mm}$ long. Find the cell's actual length and the drawing magnification.

1. One graticule division represents $100/40=2.5\ \mu\mathrm{m}$.
2. The cell length is $18\times2.5=45\ \mu\mathrm{m}$.
3. The drawing length is $36000\ \mu\mathrm{m}$, so magnification is $36000/45=800$.

Answer: Actual length $45\ \mu\mathrm{m}$; drawing magnification $\times800$.