Calibrating and measuring in microscopy – exploring onion growth

Once students have learnt how to calibrate a microscope and measure using an eye piece graticule they need practice to help embed the skills. This activity provides this practice whilst getting students to explore what happens during growth of the inner epidermis from onion layers.

The base of an onion is a very compressed stem and the layers of an onion are leaves that are modified for storage. As an onion grows, it adds new layers in the middle and the outer layers grow to accommodate the new inner layers. This means that the inner layers of an onion are young leaves that are yet to grow very big, whilst the outer layers are larger, older leaves that have been through considerable growth. By comparing layers, we can see what happens as they grow.

Do the cells get bigger as the epidermis grows, are more cells made, or both? Do the cells change in length and or width, and do their proportions change? Students can answer all these questions whilst practising their calibrating and measuring skills.