The botany student’s textbook leaf, in anatomical cross-section, is a sandwich with two thick fillings packaged between thin outer envelopes. The outer layers – upper and lower epidermis – are each usually a single layer of cells, coated with a waxy hydrophobic cuticle. The fillings – the mesophyll – have an upper rank of vertically-oriented palisade cells, packed with chloroplasts that are the seat of photosynthesis; and a lower rank of more loosely packed cells – the spongy mesophyll – with large intercellular spaces through which water and gases diffuse between the plant tissues and the surrounding air. Gases enter and exit the leaf via small pores – the stomata – in the lower epidermis; these can open and close according to the weather and the availability of water. The whole structure is supported (and supplied with water and nutrients) by a network of veins – the vascular system – that permeates the leaf and connects them to the stem and ultimately the roots.

LRB 7 February 2013 | PDF Download