18 BOTANY 
mallow or the geranium. Observe the stalk or petiole 
by which the blade is held away from the stem and 
enabled to wave freely in the air and sunshine. Next 
examine the blade itself, which, being thin and flat, 
presents a surface, large in proportion to its weight, 
for the absorption of food and sunlight from the air. 
The ribs and veins, which branch finally into a delicate 
network and are continuous with the hard tissues of 
the stem and root, not only constitute the strengthening 
skeleton of the leaf, but form the channels by which 
the leaf is supplied with much of the raw material 
needed for the formation of those compounds of which 
the tissues are built, and through which much of the 
finished product is distributed throughout the plant. 
The green colour of the leaf arises from the presence 
of a highly complex substance called chlorophyll, 
without which the energy of the sunlight could not be 
utilised. All substances in the world are either organic 
or inorganic. Organic substances are those which are 
part of, or, at one time, in their present form, were 
part of, or were produced by some living thing. Thus, 
such things as wood, milk and sugar, are organic, while 
stone, air and water are inorganic. No true animal 
ean form organic from inorganic substances. Only 
plants can do this. Thus no animal can form the 
substance of which its body is built without the help 
of plants. Plants must first form from the water, 
minerals and gases of the inorganic world, those 
organic compounds from which alone the animal ean 
build its tissues and supply itself with the energy 
necessary for its life and work. 
Animals then, receive from plants, either directly 
or indirectly, all their building material and energy. 
This brings us back again to the leaf, the factory of 
the plant, the organ in which is worked up the raw 
material from the earth and air to form these organic 
