plants, as well as those having chlorophyl, synthesize proteins, which con- 

 tain nitrogen and other elements in addition to the carbon, hydrogen and 

 oxygen derived from carbohydrates. 



Single-celled green plants carry on all the activities that together make up 

 being alive; all other cells of plants and animals depend upon chlorophyl- 

 bearing cells for food. 



Water and dissolved minerals absorbed by root hairs pass into the central 

 cylinder by diffusion; from here they move bodily to other parts of the plant 

 through special vessels; food is returned to the roots through other vessels. 



The stem of a plant is an organ of support and of transportation. Water- 

 conducting and food-conducting tubes of plant stems, as well as much of the 

 supporting tissue, are arranged in bundles. 



In monocot stems the fibrovascular bundles are scattered in the pith; in 

 dicot stems they are arranged symmetrically. 



The upward flow of water through the plant is due to osmosis in the 

 roots and between cells, and to transpiration. 



Animal life depends upon the activities of green plants; but the con- 

 tinued existence of green plants depends upon the oxidation of the organic 

 substances which in nature goes on chiefly in the bodies of animals. 



Various forms of living things are interrelated through the continuous 

 interchange of materials described as the carbon cycle, the oxygen cycle, 

 and the nitrogen cycle. 



EXPLORATIONS AND PROJECTS 



1 To demonstrate the iodine test for starch, add a few drops of iodine^ to 

 each of several test tubes prepared as follows: water only; water with cornstarch; 

 water with piece of potato; water with white flour. Use small quantities of 

 material and heat each tube to boiling. Note the blue-black color in the test tube 

 containing starch. All kinds of starches produce a similar reaction with iodine; 

 but chemists have found no other common substance that does so. We therefore 

 take a blue-black color resulting from the addition of iodine to a substance to 

 indicate the presence of starch. 



2 To show the relation of light to starch-making in leaves, expose one of 

 two healthy potted plants to sunlight and keep the other in the dark. At the end 

 of the day, remove leaves from each plant and boil them about a minute to soften 

 the tissues and to fix the starch. Then place in alcohol to remove the chlorophyl. 

 When convenient, test the leaves for starch with an iodine solution. Compare 

 results and formulate conclusions. 



3 To show the relation of chlorophyl to starch-making, use a plant with 

 variegated leaves, which have chlorophyl in some parts but not in others. After 



^Tincture of iodine may be used, or a solution of 0.3 g of iodine crystals and 0.3 g of 

 potassium iodide in 100 cc of water, 



157 



