Chapter V.

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THE LEAF.
As the root is concerned with the soil, so the leaf
is concerned with the air and sunlight. It is to these
elements in its environment that the leaf relates the
plant. It is by means of the leaf that the plant receives
carbon dioxide from the air and uses the energy of
the sunlight, first to decompose the gas and then to
build its carbon with materials from the soil into the
organic compounds the plant requires.
That the leaf is the light-related organ of the plant
is naturally suggested by the fact that, under whatever
circumstances the individual may be growing, it usually
develops in such a way as to secure for its leaves
collectively the greatest possible illumination. The
leaves, we find, lie more or less horizontally, obviously
the best position for receiving the sunlight. When,
however, plants are grown where light is received
only on one side, as is the case with window plants,
the stems turn to face the light, so that the flat surfaces
of the leaves are exposed to the rays.
Leaves again, are generally arranged on stems in
vertical rows, so that the light may reach them all by
means of the avenues between.
As a rule, the narrower the leaves, the greater the
number of rows, so that the avenues may not occupy an
undue proportion of the space. This brings us to
the question of phyllotaxis (Ck. phyllon a leaf and
taxis arrangement) by which we mean the arrangement
of the leaves on the stem. In the tea-tree, for instance,
there are five rows, in the veronica (koromiko) there
are four; while in grasses, as may be well seen in the
bamboo, there are only two. In the veronica, the
leaves are set in opposite pairs, each pair being at

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