External structure of leaves
A leaf is a lateral vegetative organ of the shoot. It carries out photosynthesis, loses water vapour (transpiration), takes part in gas exchange and in some plants also serves vegetative propagation. A leaf usually consists of a blade and a petiole; some plants also have stipules at the petiole base. Apple, apricot, poplar, walnut, grape and melon have stalked leaves; tulip, wheat, maize, barley and rice have sessile leaves, which attach to the stem by the lower part of the blade. By how the veins branch there are types of venation: parallel and arcuate (in monocots: wheat, maize), pinnate and palmate (net-like) (in dicots: apple, poplar, plane, cotton). Water and minerals come from the stem along the veins to the leaf tissue, and organic matter made in the leaf passes to the stem. Venation is an important feature for identifying plants.
| Venation | How the veins run |
|---|---|
| Parallel | veins run side by side |
| Arcuate | veins curve in arcs |
| Pinnate | one main vein with side veins |
| Palmate | several large veins fan out from the base |
Laboratory 3 (external structure of a leaf): on given leaves, herbarium sheets and house plants identify stalked and sessile leaves, look at venation under a magnifier (parallel, arcuate, pinnate, palmate), draw 4–5 kinds of leaf and label the parts. Safety: do not touch thorny or unknown plants, do not taste leaves, wash hands afterwards, and never look at the Sun through a magnifier.