252 LEAVES 



leaves lacking chlorophyll, as those of plants allowed to develop 

 in the dark, are brought to the light, chlorophyll develops. Even 

 some underground structures, as Potato tubers, will develop chlo- 

 rophyll when exposed to the sun. Hence the development of 

 chlorophyll as well as its functioning depends upon the presence 

 of light. Although the body of the chloroplast can make starch 

 regardless of the presence of pigment or light, its power to make 

 sugar depends upon the presence of chlorophyll and light. 



The veins in cross section show as colorless often glistening areas 

 in the mesophyll. In the central region of a vein are the two 

 conductive tissues, the xylem and phloem. The xylem, consisting 

 of large, empty, tube-like vessels with spiral, annular, and other 

 kinds of thickenings in their walls, occupies the upper region of the 

 vein. The xylem carries the water and mineral elements to the 

 leaf tissues. In the lower region of the vein is the phloem made 

 up of small thin- walled cells. The phloem carries away the 

 proteins and some of the sugar made by the leaves. The bundle 

 sheath, consisting of a chain of cells having large cavities and well 

 adapted to conduction, forms a sheath-like covering around the 

 vein. Through the bundle sheath much of the sugar is carried 

 away from the leaf. 



The Manufacture of Food by Leaves 



Sugar, starch, and proteins are formed in leaves, but it is the 

 manufacture of sugar that is the special function of leaves. 

 There are various kinds of sugars, but there is considerable evidence 

 that grape sugar, having the formula C 6 Hi20 6 , is the chief one 

 formed in leaves. From this sugar as a basis other kinds of sugars, 

 of which cane sugar (C^H^On) is a common one, can be formed by 

 minor chemical changes. The formation of grape sugar is a syn- 

 thetic process and, since light is necessary, the process is called 

 photosynthesis. 



Of all plant processes, photosynthesis is the most important, for 

 upon sugar as an indispensable constituent the formation of other 

 kinds of food either directly or indirectly depends. Thus without 

 the formation of sugar, such foods as starch, fats, and proteins 

 could not be formed, and consequently neither plants nor animals 

 could exist. In considering photosynthesis there are two main 

 topics: first, the nature of the process in reference to the mate- 

 rials used, the work of the chloroplasts, and the function of light; 



