PHYSIOLOGICAL DIVISION OF LABOR 67 



necessary for living. These organisms, however, live very simply. 

 The many-celled plants and animals are much more complex. 

 They consist of a variety of cells. Differentiation of cells and 

 greater specialization of function are shown in these. Whenever 

 there is a collection of similar cells in higher organisms, it is known 

 as a tissue. 



Problem. Study of the lower epidermis of a leaf. 

 Peal off the under surface of a geranium leaf. Mount it under the low 

 power of the microscope. 



I. Describe the shape, color, and structure of the different types of cells 

 in the epidermis. 



A. Epidermal cells have unusually thick walls. Remembering that the 

 epidermis is the outer layer of the leaf, suggest a function of the thick- 

 walled cells. 



II. Note the small holes between the oval-shaped cells. Each of these is a 

 stoma (from a Greek word meaning mouth or opening). Suggest a function for 

 these openings or stomata. 



A. The oval-shaped cells, called guard cells, are so constructed as to 

 cause the opening and closing of the stomata. 



B. Draw and label the lower epidermis of a leaf. 



C. Describe briefly the functions of the cells and the adaptations of the 

 cells to their functions. 



Problem. Study of a cross section of the same leaf. 

 Place a leaf between two pieces of cork and cut through the cork with a 

 sharp razor blade. 



I. Describe the general arrangement of the cells in the upper epidermis, 

 middle area, and lower epidermis of the leaf, as seen in the cross section. 



II. Note the upper colorless epidermal cells bearing hair-like structures. 

 Suggest a use for these cells. 



III. Note the regular arrangement of the cells immediately under the 

 upper epidermis. This layer is called the palisade layer or tissue. How 

 are the cells in the palisade layer arranged so as to secure sunlight ? 



IV. Observe the loose arrangement of the cells under the palisade tissue. 

 These cells form the spongy layer. 



A. What is the advantage of the air spaces among the green cells? 

 J5. State a function of the green cells of the leaves. 



