Manual of Suggestions for Teachers; To Accompany Elementary Biology, (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: PART IV. ORGANISMS IN THEIR EXTERNAL RELATIONS LXIV. OBSTACLES TO LIFE Earlier experiments have already demonstrated the influence of external forces upon growth and upon development. This chapter is in the nature of a summary with a view to a new departure. Have students make (in the form of a table) a list of ten familiar plants and ten familiar animals, with a note on the characteristic appearance or behavior (a) at high temperatures (summer) and (b) at low temperatures (winter). Have students state experiences with frostbite, frozen ear, etc., .with frozen snakes or fish, and so on. Why is freezing of protoplasm reversible, whereas the effect of heat is irreversible ? What are the dangers of natural ice? What is the object of cooling milk immediately after taking it from the cow ? What is the object of keeping milk and other food at low temperatures? What is the use of pasteurizing milk, and how is it accomplished? What is the objection to boiling milk ? In discussing the effect of water shortage or excess, point out that some of the characteristic summer and winter conditions of plants and animals are due to the water relation. What practical use is made of the fact that protoplasm cannot be active without water ? What practical methods are employed for excluding water from materials that might otherwise be destroyed by bacteria, molds, or yeast? If the influence of light upon growth has not been demonstrated earlier, it should now be shown with bacteria cultures in petri dishes. Cover a portion of each dish with black paper; or cover the whole dish and cut distinctive holes or letters in the paper, to admit light. After exposing the sterilized medium to infection, the covers are put on and the dishes are exposed to strong sunlightat room temperature or warm...