« ΠροηγούμενηΣυνέχεια »
HOW DO MOLDS LIVE AND GROW? (VIII-1)
The problem. Since green plants can manufacture foods from carbon dioxide and water they are more nearly independent than are plants which do not have chlorophyll. How do the non-green plants get their food ?
What to use. A piece of bread or stale banana, a glass plate or dish with a cover (Fig. 18), and a magnifying glass.
What to do. 1. Moisten a piece of bread slightly and expose it to the dust of the room for about an hour, then cover it to prevent drying and put it in a dark, warm place. If the banana is used treat it
Fig. 18 similarly, except it is not necessary to moisten it. Observe daily. One or more varieties of mold are likely to appear within four or five days. Examine carefully under the magnifying glass.
Questions. What was the appearance of the mold when first you could detect it? What changes in appearance occurred in the following days ? Is there any evidence that the mold consumes part of the bread or banana ? The black masses produced consist of reproductive spores. How are these spores distributed to new growing places?
Suggestions for report. Describe what was done and what occurred. State how mold plants reach new growing places.
Reference work. Read sections 92, 102, and 103. Read a description of a parasitic plant in a textbook of botany or a government bulletin ; also a description of the damage done to crops by dependent plants.
Optional problems. Do mushrooms and molds contribute to the oxygen supply of the atmosphere? Do animals? If bread or fruit which is entirely mold free is kept closed within a mold-free dish will molds develop on this bread or fruit ? How can a bread box be cared for so as to prevent the growth of molds? In what ways is the life of rusts and smuts like that of the molds ? What other plants do you know which live without the direct use of chlorophyll?
HOW DO YEAST PLANTS LIVE AND GROW? (VIII-2)
The problem. Yeasts are dependent plants, each one of which is so small that when separate it can be seen only by use of a microscope. Notwithstanding their smallness, the way in which yeasts grow has caused these plants to become of very great domestic and commercial importance. How do yeasts grow?
What to use. Cake of yeast, a sugar or molasses solution (this solution should not be stronger than 10 per cent, and a satisfactory solution can be made of corn sirup), large bottle, one-hole rubber stopper to fit, glass tubing or rubber tubing, graduate, test tube,
Fig. 19 and limewater.
What to do. 1. Arrange the apparatus as shown in figure 19. The test tube should contain limewater. Mix the yeast with a few cubic centimeters of water before adding it to the sugar solution. Observe the apparatus every two or three hours.
Questions. Does a sediment or scum form in the water ? Is there any gas formed ? Is the limewater changed? What does this show ? Has the solution an odor? What is its source ?
Suggestions for report. Draw a sectional diagram of the apparatus used, name the parts, and mention at least three products that are formed.
Reference work. Read sections 99 to 103.
Optional problems. Pour some of the fermenting yeast solution into a flask, heat to boiling for a few minutes, then close the
flask with cotton and observe from day to day. Study a drop of the yeast solution with a compound microscope. Will a 5 per
solution ferment if exposed to the air ? How do yeasts cause bread to rise ? What conditions are needed for the best work of yeast plants in bread making? What results will follow if bread rises too long?
BACTERIA IN THE AIR (IX-1)
The problem. Bacteria are extremely small plants. Most of them are much smaller than yeast plants, and it is difficult to study one or a few of them, even when the most powerful microscope is used. Yet, though the organisms themselves cannot ordinarily be seen, it is not hard to study them by observing their effects.
What to use. Some fruit juices or gelatin plates (which can usually be secured from the local public-health officer) and glass dishes with overlapping glass covers.
What to do. 1. Heat the fruit juice or the gelatin plates and the clean glass dishes to about a boiling temperature on three successive days to make certain that no living organisms remain.
2. Then expose different dishes of juice or gelatin in different ways: for example, place one in the school hallway for ten minutes ; insert a pencil point into another for one-half minute; etc. Keep one or two dishes unexposed as tests of the purity of the material used. Keep all closed dishes under the same conditions and observe from day to day. See if the liquid becomes cloudy or if colonies of bacteria develop in the gelatin. If colonies develop count them and note any differences in form, color, or effects upon the gelatin.
Questions. Do these colonies cause the decay of the fruit juice or gelatin? Are bacteria small enough to be carried by the air ? Does most air contain bacteria ? Can materials be sterilized and kept so that no bacteria will grow in them ?
Suggestions for report. State what was done in the above experiments and describe any results secured from the different exposures of the nutrient materials used. Reference work. Read sections 112 to 116.