116 Botany

properly speaking, decussate, and so appears in upright
shoots; but in creeping shoots all the leaves are brought
into the same plane by the twisting of every alternate
internode. In this way the best possible illumination
is secured.

Everywhere may be seen leaf mosaics (Fig. 79)
or groups of leaves so arranged that they receive
collectively the maximum illumination possible, or, in
other words, so placed that they throw the least possible
shade on one another.

A normal plant produces the largest leaf surface
that can be adequately illuminated, and so makes the
best possible use of the space at its disposal. This
accumultion of evidence strongly indicates that there
is a close relation between the leaf and the light.

STRUCTURE.
The external characters of the leaf have already
been examined. The stalk or petiole holds it out from
the stem, while the blade, by means of its chlorophyll,
builds the organic from the inorganic. The midrib
and veins are continuations of the vascular bundles,
that, passing from the root, traverse the stem and
branch to form a network that takes to every part of
the leaf blade the raw materials from the soil.

It will now be advisable to get some idea of the
arrangement of the tissues. Between the thumb and
forefinger of each hand hold a broad bean leaflet by
its ends, with the lower surface towards you. Now
tear it diagonally across. It will be found that a
colourless mebrane, thinner than the thinnest tissue
paper, is here and there removed. This is the lower
epidermis. On attempting to remove the upper epi-
dermis in the same way it will be found that a certain
amount of the green tissue from the interior comes
away with it. Where both upper and lower epidermis
have been removed it will be seen that between