WORK OF THE BEAN LEAF 275 



LABORATORY KXTKIIIMKN IS 



Hold the underside of a geranium Leaf against a cool window pane and 



note the moisture which is condensed. Try other leaves in tin- .sun.- way. 

 With clips fasten a watch crystal to a growing leaf ami seal with vaseline. 

 Note the moisture condensed. Try the upper side of the saim- lean 

 Plunge a leaf into water and set the water in the sun. Do small 

 bubbles appear on the surface of the leaf ? Where ? 



Take leaves of the same plant and coat with paraffin one leaf on both 

 sides, another on the upper side, and a third on the underside. Lay them 

 aside for a few days. Then remove the paraffin and examine all the 

 leaves. Which is in the best condition ? Why ? 



Examine with a microscope the epidermis of a number of leaves bom 

 different plants. Note the irregular epidermal cells and the stomata cells. 

 Are the stomata arranged regularly? 



Hold a leaf up to the light and notice the arrangement of the veins 

 and soft parts. Study a cross section of a fresh leaf and find : (1) the epi- 

 dermal layer on top ; (2) the palisade layer below it ; (3) the wide, spongy 

 layer ; and (4) the lower epidermal layer with stomata. 



Stand the petiole of a leaf in red ink and observe how the color spreads 

 through the veins of the leaf. 



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210. The Work of the Bean Leaf. — • As soon as the bean 

 plant gets its plumule into the air, the pale leaves unfold, 

 turn green, and increase in size. The stem elongates, 

 branches, and other leaves appear. Bach of these new 

 leaves is held out from the stem or branch in a position 

 which gives the leaf the greatest possible amount of air 

 and light. The leaves of the plumule begin to be useful 

 to the plant as soon as they become green. Their work 

 is most important in the life processes of the plant. 



Does the bean plant respire? When an animal respires, 

 it takes oxygen into the cells of its body ami gives off 

 carbon dioxide. The presence of this gas is shown by 

 forcing: the air that comes from the Lungs through a tube 

 into limewater. The limewater becomes cloudy. This i> 



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a simple chemical test for carbon dioxide. 



If a growing bean plant is kept tightly covered under a 

 glass disk for twenty-four hours and then the inclosed air 



