54 A FIELD GUIDE IN NATURE-STUDY 



Heat the needle in a flame to kill any spores that might be on it. Then 

 draw the needle through the spore-bearing black mold or the green mold, 

 trying to get spores of one kind only on it. Open the jar containing the 

 sterilized bread and quickly draw the needle across the surface of the bread. 

 Do not leave the jar open longer than is necessary. When it is closed 

 tightly again let it stand a few days. Where does mold begin to appear 

 on the bread? What kind is it? What is your conclusion? What is 

 essential that any process of sterilization be effective ? 



Useful fungi. The molds and other fungi are among man's most useful 

 allies, although. some of them are also injurious. The soil, especially rich 

 soil like that of the forest, is full of them, and they decompose the organic 

 matter and reduce it to substances that plants can use for food once more. 

 The very numerous hyphal threads come in contact with the roots and 

 help them to get in touch with adequate supplies of moisture and nutrition. 

 Examine rich garden soil or humus from the woods to see if you can detect 

 these molds. 



Yeast. One sort of yeast is used constantly in bread-making and other 

 kinds are used in the manufacture of beer, cheese, etc. The compressed 

 yeast cake is just a mass of these yeast plants with more or less starch and 

 other foreign matter. What is its color ? The dried yeast cake is the same, 

 dried out. Put a quarter of a yeast cake into a half-cup of molasses or 

 sugar syrup in a flask or bottle and let it stand in a warm place until it 

 ferments. The sugar is being used in part by the growing yeast plants for 

 food, but certain wastes are given off. One is alcohol, the odor of which is 

 readily detected. Another may be demonstrated by fitting the mouth of 

 the bottle or flask with a cork from which a delivery tube passes into a test 

 tube filled with limewater. What is the result and what process does this 

 show is going on in the growing yeast ? Why does the yeast have to be 

 provided with organic food like sugar ? 



Bacteria. Many more of the plants most significant to man are exceed- 

 ingly tiny. Such are the bacteria, some of which are very useful, like the 

 nitrifying bacteria on the roots of the leguminous plants or those that live 

 in the human intestines and help with the digestive processes, while others 

 are injurious, such as those that cause tuberculosis. Draw a circle three 

 inches in diameter to represent the cross-section of a human hair. Hairs 

 vary in diameter, some persons having very coarse, others very fine, hair, 

 but one four-hundredths of an inch would be an average diameter. A dot 

 with a sharp pencil will represent the diameter of a tubercle bacillus and a 

 line one-eighth inch long its length on the same scale. 



Rate of reproduction. Such bacteria are too small to be seen Except 

 under very powerful microscopes, but they multiply rapidly by repeated 



