THE WORK OF LEAVES 201 



the other soil -food (water and the substances dissolved 

 in it) . These two kinds of raw, or crude food meet in 

 the leaf, and there unite to form elaborated food, 

 i. e., sugars, starch, fats, oils, proteids, etc. 



The leaf absorbs not only food, but energy, i. e., 

 sunlight, which is needed to manufacture elaborated 

 food. The absorption of light is the work of the green 

 coloring matter, the chlorophyll, and especially of the 

 palisade cells, which are on the upper side of the leaf, 

 directly exposed to the light. If we examine a section 

 of a leaf which exposes both sides equally to the light 

 (such as Iris, Gladiolus, etc.), we find palisade cells 

 on both surfaces (Fig. 114) . In many leaves there is 

 but a single row of palisade cells, while in others we 

 find a double row (Fig. 113) , which results in the more 

 complete absorption of light. The ability of leaves to 

 absorb light is easily tested by making a tube (about 

 an inch in diameter and twelve inches long) of card- 

 board, directing it toward the light, placing a leaf over 

 the end and looking through it. Find out how many 

 thicknesses of leaf are necessary to 

 absorb the light completely. 



Within the tiny chlorophyll gran- 

 ules, the chlorophyll is scattered in 

 the form of very minute drops (Fig. 115. A single chlorophyll 



grain containing very 



115) , which arrangement enormously 

 increases the absorptive surface of 



, , in T j i -A_ the form of very mi- 



the chlorophyll, and consequently its nut* drops. 



