THE DIFFERENTIATION OF THE PLANT-BODY 21 
would largely deprive it of water. Hence in plants with 
roots distributed in this way we find devices to conduct 
the water into the centre of the mass of the shoot system. 
When we pass to a closer examination of the much 
divided or branched shoot we almost invariably find that its 
ultimate twigs put forth certain regularly arranged flattened 
expansions. In cases where there is much exposure to 
currents of air these flattened portions are furnished with 
stalks of variable length, which are extremely flexible and 
allow the flattened organs to sway freely backwards and 
forwards ag the wind blows upon them. These flattened 
portions further are usually of a vivid green colour. They 
are known as leaves, or preferably foliage leaves. 
As almost all plants possess leaves, we may inquire 
why these organs should so uniformly be thin and flat. 
There are several reasons of almost equal importance. 
The leaf or other winged part of the shoot is in contact or 
relation with the air only. Interchanges of gases between 
the air and the leaf are continually going on, and these 
interchanges are effected most easily and fully by means 
of a large extent of surface. No form gives so much surface 
in proportion to its bulk as a thin flat plate, just such a 
form indeed as the flattened portion or blade of the leaf. 
The interchanges include the absorption of particular gases 
from the air, and the giving out of gases and water vapour. 
As we shall see later, the internal structure of the leaf-blade 
is arranged largely with a view to the carrying out of these 
exchanges. 
A second reason for the flattening of the leaf is concerned 
with the manufacture of the plant’s food. A particular 
gas known as carbon dioxide, which is taken in from the 
air, is ultimately built up into a true food, a kind of sugar. 
Though the formation of sugar in the plant is only partly 
understood, it is known to depend upon the presence of 
the green colouring matter and upon its being properly 
illuminated. The flattened form helps to expose the green 
pigment to the light to the greatest advantage. 
