INTRODUCTORY 13 



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 then known as leaves, or, pre- 

 ferably, 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 portion 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 with 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 absorp- 

 tion 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 

 material, a kind of sugar. Though the formation of 

 sugar in the plant is not fully understood, it is known to 

 depend upon the presence of the green colouring matter 

 and its being properly illuminated. The flattened form 

 helps to secure the arrangement of the green colouring 

 matter in such a way that the light, either of direct 

 sunshine or of the less bright diffused daylight, may 

 reach it with the least obstruction. 



Yet a third reason may be given. The leaves are 

 very frequently so placed that they extend outwards 

 from the plant and lie nearly parallel to the surface of 



