192 



CIVIC BIOLOGY 



compressed yeast contains approximately 10,000,000,000 of 

 them. In order, once for all, to gain a notion of the minuteness 

 of microorganisms, perform the following simple experiment : 



Sharpen the point of a teasing-needle to a tine knife bhide ; lake a 

 bit of moist compressed yeast, the size of a large pinhead, on a piece of 

 clean, polished glass (a microscope slide) and cut the lump in halves. 

 Throw away one half and repeat the operation and continue as long as 

 you can see to divide the particle. At the last division carefully plant 

 one half in a vial half full of filtered, boiled molasses and water (a table- 

 spoonful of molasses in half a pint of potato water makes a good cul- 

 ture fluid), to watch it grow from day to day. Then, with the point of 

 a clean needle, on a perfectly clean part of the glass, cover the other half 



with a minute droplet of 

 water. Cover with a per- 

 fectly clean cover glass 

 and try to count the tor- 

 ulae (yeast plants) in the 

 speck that you can just 

 see with the naked eye. 



Fig. 93. Yeast plants, highly magnified, show- 

 ing successive stages of growth by budding 



After Conu 



In color most of the 

 common yeasts, when 

 seen in mass, are whit- 

 ish or slightly yellowish gray, the color of a fresh yeast cake, 

 but a few species are pink, red, or black. 



Distribution. Y^easts are everywhere ; so the question is 

 not, Where shall we go to find them, but. Where go to escape 

 them ? We eat them by billions, baked, in our daily bread ; 

 we drink them by millions, alive, in our eider, beer, or Avine : 

 we breathe them in, alive, with e^ery breath, and drink them, 

 alive or dead, according as the water is raw or ])oiled, with 

 every drink of water we take ; they are all over us all the 

 time, in our hair, on our skins, in all our clothes, and we 

 cannot possibly beat them out, brush them off, or e*^^en wash 

 them away — the harmless, useful, patient, persistent, omni- 

 present little sugar-hunting yeast plants. We might suck 



