278 EXPEEIMJ^NTS ]VITH PLANTS 



The intimate connection between the work of the 

 stem and that of the leaf, which we have had occasion 

 to note frequently in this chapter, is further empha- 

 sized by the fact that in most cases the stem (in the 

 younger portions at least) contains chlorophyll and 

 shares in the work of starch-making. 



Does the stem use up oxygen and produce carbon 

 dioxide, as germinating seeds, roots and leaves do? 

 Repeat the experiment described on page 194, using 

 pieces of stems instead of leaves. 



How does the stem obtain the necessary supply of 

 air for these processes? Do you find stomata in the 

 epidermis ? 



Strip off a piece of the epidermis and examine it 

 for stomata (as described on page 196). Examine 

 also a thin section in water, and observe the bubbles 

 of air between the cells (under the microscope they 

 have a very characteristic dark appearance). Ex- 

 amine an older part of the stem (where the color is 

 no longer green) in the same way. Attach a bicycle 

 pump to the stem by a short section of thick, white 

 rubber tubing (which should be secured at the joints 

 by elastic bands or wire), close the free end of the 

 stem with sealing-wax, place the stem under water 

 and pump air into it (Fig. 154). The openings, 

 which are visible to the naked eye and which (in the 

 Birch, Cherry, etc.) may become an inch or more 

 long, are known as lenticels. A section through one 



