Left L. P. f: - 



Porous cup 

 Water 



Water 



Mercury 



Inj'.itute 



If we cut the stem of a living 

 plant under cold water that has 

 been boiled to remove the air, 

 and then connect it with a glass 

 tube while still under water, the 

 vessels of the stem and leaves 

 are in communication with the 

 water in the tube. Now we may 

 set the stem upright, with the 

 lower end of the tube dipping 

 into mercury. In this arrange- 

 ment mercury rises in the tube 

 as if the water were being pulled 

 or pushed into the stem. With a 

 porous cup full of water in place 

 of the twig, the water and mercury 

 behave in the same way. What 

 becomes of the water that dis- 

 appears out of the glass tube? 

 How is the water actually raised? 



WATER RAISED BY TRANSPIRATION 



the soil with its dissolved salts — but not in the bark or phloem vessels. The 

 following spring, however, the buds will not open; the tree will be dead. 

 This is because the water now coming from the roots is without organic 

 food. The food reserves could not come do\Mi into the roots after the tree 

 was girdled, for it is through the phloem vessels that organic food comes 

 from the leaves to the lower parts of the plant. 



Is There Danger of Exhausting the Supply of Raw Materials Used 



by Plants in Food Production? 



The Carbon Cycle If we understand how green plants make food, 

 we can see more clearly how the living things in the world depend upon 

 each other. The carbon in our bodies, for example, came from the proteins, 

 fats and carbohvdrates which we ate. We obtained these either from the 

 bodies of plants or from the bodies of animals. The cows or pigs or 

 chickens that we used as food had in turn obtained the carbon in their 

 bodies from the plant food which thev had eaten. 



Now die plant gets its carbon from the carbon dioxide in the air. But 

 what is the source of this fraction of 1 per cent of the atmosphere.^ The 

 plants in North America could use it all up in a few sunny August days — 

 and that would be the end of everything. Certain rocks — limestone and 

 marble especially — yield small quantities of this gas when thev decompose. 



148 



