CHAPTER V. 
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 
earbon 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 
circumstanees 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 lght 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 (Gk. 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 heing at 
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