HOW PLANTS DO THEIR WORK 295 



does this show as to where transpiration takes place more 

 rapidly ? 



We are now ready to find out something about the way 

 in which plants manufacture their products. To under- 

 stand this we must know something about leaves. 



Examine a leaf of any plant. What are its parts — 

 stem or petiole, expanded portion or blade. Hold the leaf 

 between you and the light. What do you see in it ? How 

 are the veins arranged in a bean leaf, maple, sunflower ? 

 These are net- veined leaves. How are the veins arranged 

 in a corn leaf? In grass? These are parallel veins. What 

 is the use of the veins? By discussion the value of the 

 veins in holding the leaf spread will be brought out. 

 Place a twig with growing leaves in a tumbler of water 

 colored with red ink. After twenty-four hours examine 

 the petiole, the veins. What do you conclude? If you 

 can procure a thick leaf, as live-for-ever, tulip, or hepatica, 

 have the pupils peel off a little of the skin or epidermis. 

 Even a thin leaf may have a little of the covering removed, 

 enough to lead the pupils to see that the entire leaf is 

 covered with a thin, almost transparent skin. What is 

 under the skin? This green, granular mass is chiefly 

 chlorophyll bodies. If the pupil could see a cross section 

 of a leaf highly magnified he would find it built up of cells 

 one layer above another. Each cell has a thin wall and 

 contains a number of green, roundish bodies called chloro- 

 phyll bodies, and a mass of colorless protoplasm. The 

 protoplasm is the living part of the leaf, and is the machin- 

 ery that manufactures the plant products. But just as 

 any machinery must have power to make it run, so must 



the protoplasm of the leaf. What is the power ? 

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