116 2. 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 
accumulation 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 cut from 
the stem, while the blade, by means of its ehlerophyll, 
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 membrane, thinner than the thinnest tissue > 
paper, 1s 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 
