74 AGRICULTURE ON THE PACIFIC SLOPE 



iodine solution. 1 If the leaves have much starch in 

 them, they will turn dark in color. Now, by means of 

 aj3Q&..or piece of cloth or paper, cover up the plant 

 from which the leaves were taken. Try to shut off 

 the light from them as much as you can. After two 

 or three* clays remove the box and test the leaves for 

 starch as described above. If they contain no starch, 

 it shows that what starch they had at the beginning 

 has gone into the stem to help it to grow, and that they 

 cannot make starch without sunlight. 



Can the leaf make starch without air? Some of these 

 leaves which have no starch in them may be used to 

 answer this question. Place vaseline on the upper 

 surface of some of the leaves, smearing it on thickly 

 enough to keep out all the air. Cover other leaves 

 on the lower surface only. After two or three days 

 test for starch in the way given above. 



If we find that covering the lower surface has more 

 effect than covering the upper, we may be sure that it 

 is because the air can enter the leaf more easily from 

 the lower side. 



Air enters the leaf and passes down into the stem 

 so that both leaf and stem may breathe. Make a small 

 hole in a cork (or, better, in a rubber stopper) and force 

 the end of a leafy branch through it as shown in figure 



1 This can be made (by a druggist, if necessary) by dissolving 

 potassium iodide in water (1 part to about 75 of water) and adding 

 crystals of iodine until the color is dark brown. 



