BIOLOGY FOR YOUNG BEGINNERS. 



575 



To begin with, you must have a spoonful or two of yeast to look 

 at while I talk. You will probably notice first a number of bubbles, 

 like soap-bubbles ; but, instead of common air, the yeast-bubbles con- 

 tain a gas made by the yeast carbonic-acid gas. Next you will no- 

 tice the brownish color of the yeast ; it grows thicker and muddy, and 

 after an hour or so begins to rise. This rising the chemists call fer- 

 mentation ; biologists call it growing. The spoonful has become a 

 cupful. The yeast is really alive, and it is one of the simplest forms 

 of life. In studying biology, then, or the science of life, we begin 

 at what seems the beginning. 



All that I have described you can see with your own eyes ; but 

 now I must tell you something about the yeast which you could never 

 find out with your eyes alone. With the aid of the microscope, a 

 great many little solid bodies are seen floating about in it. Some- 

 times they are found alone, but most frequently in groups (Figs. 1-5), 



O o 



Fig. 1. Round Cells. 



Fig. 2. Lemon-shaped Cells. 



Fig. 3. Outer and Inner 

 Surface of Sac. 



Fig. 4. Group of Cells. 



Fig. 5. Group of Cells. 



and each one is about i0 \ of an inch in size. Though they are solid, 

 yet we can see through them, and they are always round (Fig. 1), 

 some of them not quite as round as a ball, more like a lemon (Fig. 

 2), but none of them are square or flat. The cover of each one is 

 double (Fig. 3), that is, it has an outside and an inside. Under the 

 microscope, these two surfaces look like two round lines, one within 

 the other (Fig. 6). Inside these lines is something which looks like 

 little grains (Fig. 1) ; and this whole cover, with all that is inside of 



Fig. 6. 



Fig. 7. 



Fig. 8. 



Fig. 9. Yeast-cell, or Torula. 



a. .Telly, or Protoplasm ; 



b, Thin Space, or Vacuole. 



it, is called a cell. Now you must learn of what these cells are made. 

 First there is the outside part, which is like a bag or sac. This bag 

 is tough and solid, and is full of a jelly-like substance, which is thick 



