CALIFORNIA PLANTS IN THEIR HOMES 



ous germs. In general, a study of these plants has proved 

 that cleanliness and care can prevent many diseases. 



But even bacteria are not all bad. When plants and 

 animals die, it is the bacteria that cause their decay; that 

 is, this multitude of invisible plants, in getting their own 

 food from dead organic matter, break it up. In taking 

 what they need for their own growth, they set free valuable 

 substances that would otherwise remain locked up. Car- 

 bonic acid gas is returned to the air, and the nitrogen that 

 formed part of the dead plants and animals, is prepared by 

 the bacteria so that it can be used by living plants. So 

 finally all the dead matter is used up and disappears, leaving 

 room in the world for new generations of plants and animals. 



There are other invisible plants, almost as small as the 

 bacteria, that make our bread light and edible. When we 

 stir tiny yeast plants into flour and water and keep them 

 warm, they become active at once, and eagerly help them- 

 selves to the food that the wheat made for its own seed- 

 lings. Bread is raised by the bubbles of gas that the yeast 

 plants set free, as they take what they want for themselves. 

 It should be baked in time to kill the little plants before 

 they rob it of so much food that it is sour. In like manner 

 minute plants cause what we call alcoholic fermentation in 

 beer and in wine and fruit juice generally. When fruit is 

 canned it is heated in order to kill all such mischief-makers. 



But there are also vagabond plants that we can see 

 without the microscope. Keep some bread moist and warm 

 for a few days and watch it closely. You will see first a 

 mass of fuzzy, white hairs; soon you will be able to make 

 out clusters of tiny white stalks with balls on the ends; 

 finally you will see the balls turn black and the whole mass 

 darken. Your bread will then be covered with a ripened 

 crop of plants called black mould. One of the clusters 

 under the microscope looks like No. i, Fig. 17. You see 

 there are root-like cells for sucking in the ready-made 



52 



