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 covering of the leaf. 

 About what is the ratio of the number of 

 stomata to the number of epidermal cells in 

 a small part of the leaf? Draw one or two 

 stomata showing all parts, as seen under a 

 low power of the microscope. If we now 

 examine a very thin piece of a leaf cut in 

 cross section, we can make out the rela- 

 tion of the stomata to the interior of the 

 leaf. 



Lahoratorij Exercise on Cross Section of the Leaf. — In a cross section of 

 the leaf of Tradescantia, or any lily, find under the upper epidermis 

 a layer of green cells closely packed together (called collectively the 

 palisade layer). These cells are more or less columnar in shape. Under 



1 See Hunter and Valentine, Manual, page 243. 2 jfji^., page 242. 



hunter's BIOL. — 9 



Surface view of epidermis of lower 

 surface of a leaf; e, ordinary epi- 

 dermal cell; g, guard cell. — 

 Tschirch. 



