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 as 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. 



