. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. LEAVES AND THEIR FUNCTIONS 129 leaves entirely green, for several days in a well-lighted window under the same conditions of light, heat, and moisture. After two days, pick several leaves from each plant and place them in separate jars in wood alcohol which will extract the leaf green. When the leaves are entirely cohjrlcssi test the two sets of leaves witii tincture of iodine. Which leaves contain starch? Which parts of the leaf contain starch ? This ex- periment shows us that starch is pr
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. LEAVES AND THEIR FUNCTIONS 129 leaves entirely green, for several days in a well-lighted window under the same conditions of light, heat, and moisture. After two days, pick several leaves from each plant and place them in separate jars in wood alcohol which will extract the leaf green. When the leaves are entirely cohjrlcssi test the two sets of leaves witii tincture of iodine. Which leaves contain starch? Which parts of the leaf contain starch ? This ex- periment shows us that starch is present only in the green part of the leaf.^ Relation of Starch Forma- tion to Sunlight. — Another simple experiment will give us the relation of the presence of starch to the sunlight. Pin or sew several strips of black cloth, such as alpaca, over the leaves of a growing geranium. Place the plant in a sunny window for several days. (This can be reduced to hours if the plant has pre- viously been kept in the dark for a day of two.) If we now. Starchless areas in leaves, caused by excluding sun- light with strips of black cloth. extract the leaf green as before, and then test with iodine, we find that starch is present only in that part of the leaf which was exposed to the sun. A green leaf, when attached to the plant, and under natural conditions, forms starch in the sunlight.^ Examination of the Under Surface of a Leaf Under the Microscope. — Strip off the under surface of a leaf of Tradescantia, stretch it flat in water on a glass slide and examine it with a good hand lens (or, better, the low power of a com- pound microscope); numbers of little oval structures will be seen. These are called stomata (singular stoma). Notice the two cells, usually kidney-shaped, one on each side of the stoma. These are the guard cells. By change in shape of these cells the open- ing of the stoma is made larger or smaller. Note also the larger irregular cells of the epidermis or outer coverin
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