8 



Elementary Principles of Agriculture 



yeast, except that their sides are flattened by pressing 

 against each other. New cells are formed by a single 

 cell dividing into two cells (Fig. 6). These new cells 

 grow to a certain size and divide again, and so on till 

 great numbers are formed. (See Fig. 14, C.) 



14. The Living 

 Substances of Cells. 

 The cell is the unit 

 out of which all plant 

 and animal bodies are 

 made, just as the 

 brick is the unit out 

 of which buildings are 

 made. Within each cell-wall is the living substance, 

 called protoplasm. It differs from dead substance in 

 that it has a different chemical constitution, and the 

 power of self-action. Protoplasm is a clear granular 

 substance, like the white of an egg or mucilage. It 

 differs from these in that it has life. 



Fig. 6. In forming new cells the living sub- 

 stance orpotoplasm divides and then a cell- 

 wall is formed between them. 



QUESTIONS 



1. Define environment. 2. What is the purpose of "culture?" 



3. What is the most important condition of plant environment? 



4. Describe the yeast plant. 5. Name other kinds of these simple 

 plants, and mention their importance. 6. What do you learn from 

 the yeast experiment as to the kind of food used by the yeast plants? 



7. What is the chief difference between a fungus and an alga? 



8. What do you learn from the "pond scum" experiment as to 

 the food of the algae? 9. Are the higher plants, such as herbs and 

 trees, in any way similar to simple plants, such as yeast and pond 

 scum? 10. Why are green plants called independent; fungi, de- 

 pendent plants? 



