268 



PHOTOGRAPHY 





sun. But nearly all common plants, whatever they are, sicken 

 and die if deprived of sunlight for a long time. This is likewise 



true in the animal 

 world. During long 

 transportation, animals 

 are sometimes necessa- 

 rily confined in dark 

 cars, with the result 

 that many deaths oc- 

 cur, even though the 

 car is well aired and 

 ventilated and the food 

 supply good. 



In addition to the 

 plants and animals which we see, there are many strange unseen 

 ones floating in the atmosphere around us, lying in the dust of 

 corner and closet, growing in the water we drink, and thronging 

 decayed vegetable and animal matter. Every one knows that 

 mildew and vermin do damage in the home and in the field, but 

 very few understand that, in addition to these visible enemies 

 of man, there are swarms of invisible plants and animals some of 

 which do far more damage, both directly and indirectly, than 

 the seen and familiar enemies. All such very small plants and 

 animals are known as microorganisms. Not all microorganisms 

 are harmful ; some are our friends and are as helpful to us as 

 are cultivated plants and domesticated animals. Among the 

 most important of the microorganisms are bacteria, which in- 

 clude among their number both friend and foe. Although sun- 

 light is essential to the growth of most plants and animals, it 

 destroys bacteria. 



Light and growing plants. All plants, except a few like 

 bacteria and molds, need light for growth and development. 

 This is largely because light by its chemical action enables 



