LIGHT AND LIFE. 105 
gen retained by the plant is replaced by nitrogen thrown 
off; and supposed that this nitrogen was furnished by the 
substance of the plant itself. This function of the green 
portions of vegetables is, moreover, performed with great 
rapidity and energy. Boussingault, who has made some 
remarkable experiments on this subject, filled a vessel of 
water with vine-leaves, placed it in the sun, and sent acur- 
rent of carbonic acid through it; on its passing out, he col- 
lected nothing but pure oxygen. It is calculated that a 
leaf of nenuphar gives out in this way during the summer 
more than sixty-six gallons of oxygen. 
In 1848 Cloéz and Gratiolet contributed new facts. 
They showed that aquatic plants follow the same course 
during the day as others, but that at night they are at rest, 
and give rise to no release of carbonic acid. They proved 
the powerful, instantaneous action of solar light on vege- 
table respiration. If a few leaves of potamogeton or of 
nayas are put into a gauge full of water saturated with 
carbonic acid, as soon as the apparatus is placed in the sun, 
an immense number of light bubbles, of almost pure oxygen, 
are seen to detach themselves from the surface of the leaves. 
The shadow of a slight cloud, crossing the sky, suffices to 
check their disengagement at once, followed by sudden 
activity after it has passed. By intercepting the solar 
beam with a screen, the alternations of quickness and slow- 
ness in the production of gas-bubbles may be very plainly 
seen, according as the plant receives the rays or not. Wa- 
ter-plants show other interesting peculiarities. Diffused 
light has no power to excite the production of carbonic 
acid, unless the phenomenon has been first called forth by 
direct sunlight. Still further, the solar influence having 
once been applied, the evolution of carbonic acid continues 
even in darkness. ‘The vegetable keeps up at night its 
mode of breathing by day. The living force of solar light, 
therefore, can be fixed and stored away in living plants, as 
