as 70% air by volume. This structure helps
explain why leafy vegetables shrink so much
when cooked: heat collapses the spongy
interior. (It also wilts the leaves so that they
pack together more compactly.)
Cross section of a leaf. Because
photosynthesis requires a continuous supply
of carbon dioxide, leaf tissue often has a
spongy structure that directly exposes many
inner cells to the air.
An exception to the rule against storage
tissue in leaves is the onion family (tulips and
other bulb ornamentals are exceptions as
well). The many layers of the onion (and the
single layer of a garlic clove) surrounding the
small inner stem are the swollen bases of
leaves whose tops die and fall off. The leaf