ll 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 the
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
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.

[Illustration: FIG. 85--Stems and leaves of oxalis growing toward the
light.]

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. Everyone 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 microoerganisms 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 microoerganisms are bacteria, which include
among their number both friend and foe. In the household, bacteria are
a fruitful source of trouble, but some of them are distinctly friends.
The delicate flavor of butter and the sharp but pleasing taste of
cheese are produced by bacteria. On the other hand, bacteria are the
cause of many of the most dangerous diseases, such as typhoid fever,
tuberculosis, influenza, and la grippe.

By careful observation and experimentation it has been shown
conclusively that sunlight rapidly kills bacteria, and that it is only
in dampness and darkness that bacteria thrive and multiply. Although
sunlight is essential to the growth of most plants and animals, it
retards and prevents the growth of bacteria. Dirt and dust exposed to
the sunlight lose their living bacteria, while in damp cellars and
dark co