CoachA stoma is a pore in the skin of a leaf, bounded by two guard cells that open it by swelling and close it by shrinking. Through the open stomata carbon dioxide enters for photosynthesis and water vapour leaves — the transpiration that a plant cannot avoid while it feeds. A leaf carries one to three hundred stomata per square millimetre, mostly on its lower surface; they open in the light and close in the dark and in drought.
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Example 28.4 (The numbers of a tree)
A mature birch on a summer day transpires of water, lifted through xylem tubes a tenth of a millimetre wide at per hour. Less than 1% of that water is kept for photosynthesis; the rest is the price of open stomata. In return the leaves fix some of carbon dioxide into of sugar, of which the phloem delivers a share to every root tip and every ripening seed.