85
See also: Joseph Black 76–77 ■ Henry Cavendish 78–79 ■
Joseph Priestley 82–83 ■ Joseph Fourier 122–23I
n the 1770s, Dutch scientist Jan
Ingenhousz set out to discover
why plants, as earlier scientists
had noticed, put on weight. He went
to England and did his research at
Bowood House—where Joseph
Priestley discovered oxygen in
1774—and was about to find the
keys to photosynthesis: sunlight
and oxygen.Bubbling weeds
Ingenhousz had read how plants in
water produce bubbles of gas, but
the bubbles’ precise composition
and origin were unclear. In a series
of experiments, he saw that sunlit
leaves gave off more bubbles
than leaves in the dark. He collected
the gas produced only in sunlight,
and found that it re-lit a glowing
splint—this was oxygen. The gas
given off by plants in the dark put out
a flame—this was carbon dioxide.
Ingenhousz knew that plants put
on weight with little change in the
weight of the soil they grew from.
In 1779, he correctly reasoned that
gas exchange with the atmosphere,
especially the absorption of the gascarbon dioxide, was at least partly
the source of a plant’s increased
organic matter—that is, its extra
mass came from air.
As we now know, plants make
their food by photosynthesis—
converting energy from sunlight into
glucose by reacting the water and
carbon dioxide that plants absorb,
and releasing oxygen as waste. As a
result, plants supply both the oxygen
that is vital to life, and—as food for
others—the energy. In a reverse
process called respiration, plants
use the glucose as food and release
carbon dioxide, day and night. ■EXPANDING HORIZONS
THE MASS OF A
PLANT COMES
FROM THE AIR
JAN INGENHOUSZ (1730–1799)
IN CONTEXT
BRANCH
Biology
BEFORE
1640s Flemish chemist Jan
Baptista van Helmont deduces
that a potted tree gains weight
by absorbing water from soil.
1699 English naturalist John
Woodward shows that water is
both taken in and given off by
plants, so their growth needs
another source of matter.
1754 Swiss naturalist Charles
Bonnet notices that plant
leaves produce bubbles of air
under water when illuminated.
AFTER
1796 Swiss botanist Jean
Sénébier shows that it is the
green parts in plants that
release oxygen and absorb
carbon dioxide.
1882 German scientist
Théodore Engelman pinpoints
chloroplasts as the oxygen-
making parts in plant cells.
Pondweed bubbles at night show
respiration as plants convert glucose
into energy, absorbing oxygen and
releasing carbon dioxide.