132 



PHOTOGRAPHY 



and grow. A healthy plant brought into a dark room soon 

 loses its vigor and freshness, and becomes yellow and droop- 

 ing. Plants do not all 

 agree as to the amount 

 of light they require, 

 for some, like the violet 

 and the arbutus, grow 

 best in moderate light, 

 while others, like the 

 willows, need the 

 strong, full beams of 



FIG. 85. Stems and leaves of oxalis growing the SUn. But nearly all 



common plants, what- 

 ever 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 necessarily confined in 

 dark cars, with the result that many deaths occur, even though 

 the car is well aired and ventilated and the food supply 

 good. Light and fresh air put color into pale cheeks, just as 

 light and air transform sickly, yellowish plants into hardy 

 green ones. Plenty of fresh air, light, and pure water are 

 the watchwords against disease. 



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 ver- 

 min do damage in the home afid 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. 



