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 sugar, but there is considerable evidence 
that grape sugar, having the formula CsH,20¢, is the chief one 
formed in leaves. From this sugar as a basis other kinds of sugar, 
of which cane sugar (Ci2H».O,1) 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; 
