Perhaps the most striking thing about tree leaves is their tremendous diversity in size. The Arctic–alpine snow willow (Salix nivalis), which grows around the northern hemisphere, can have leaves just 4 mm long on a sprawling ‘tree’ no more than a centimetre high (Figure 2.1). Smaller still, the scale needles of some cypresses are nearer a millimetre long. Among the largest of leaves are those of the foxglove tree (Paulownia tomentosa), which on coppiced trees can be over half a metre in length and width on a stalk another half metre long. Such large sail-like leaves are in great danger of being torn by the wind (as in the traveller's palm, Ravenala madagascariensis; see Figure 2.1) so it is perhaps no surprise that big leaves are usually progressively lobed and divided up into leaflets to form a compound leaf. This can lead to even larger leaves: the Japanese angelica tree (Aralia elata) can have leaves well over a metre in length (Figure 2.1). Many palms have feathery leaves over 3 m long and in the raffia palm (Raphia farinifera) up to 20 m (65 feet) long on a stalk another 4 m long.

The leaves are the main powerhouse of the tree. Combining carbon dioxide from the air with water taken from the soil they photosynthesise, using the sun's energy to produce sugars and oxygen. These sugars (usually exported from the leaf as sucrose, the sugar we buy in packets) are the real food of a tree.