Physical geography. By Mary Somerville ...

CHAP. XXIIT. NOURISHM-IENT OF PLANTS. 323
flowering and forming of the fruit, become by far the most abundant; and a set of invisible rays, which exist near the point of maximum heat in the solar spectrum, are also most abundant in summer.
Mr. Hunt found that the hot rays immediately beyond the visible
red destroy the colour of certain leaves; and for that reason the
glass of the great palm-house at Kew Gardens is tinged pale yellowgreen, to exclude the scorching rays in question, though it is permieable by the other rays of heat, those of light, and the chemical
rays.
In spring and summer the oxygen taken in by the green leaves in
the night aids in the formation of oils, acids, and the other parts
that contain it; but as soon as autumn comes, the vitality or chemical action of vegetables is weakened; and the oxygen, no longer
given out in the day, though still taken in during the night, becomes
a minister of destruction; it changes the colour of the leaves, and
consumes them when they fall. Nitrogen, so essential during the
life of plants, also resumes its chemical character when they die, and
by its escape hastens their decay.
Although the food which constitutes the mass of plants is derived
principally from the water and the gases of the atmosphere, fixed
substances are also requisite for their growth and perfeution, and
these they obtain from the earth by their roots. The inorganic mat
ters are the alkalis, phosphates, silica, sulphur, iron, and others.
It has already been mentioned that vegetable acids are found in
the juices of all the families of plants. They generally are in combination with one or other of the alkaline substances, as potash, lime,
soda, and magnesia, which are as essential to the existence of plants
as the carbonic acid by which these acids are formed: for example,
vines have potash; plants used as dyes never give vivid colours without it; all leguminous plants require it, and only grow naturally on
ground that contains it. None of the corn tribe can produce perfect
seeds unless they have both potash and phosphate of magnesia; nor
can they or any of the grasses thrive without silica, which gives the
1 The solar spectrum, or coloured image of the sun, formed by passing
a sunbeam through a prism, is composed of a variety of invisible as well
as visible rays. The chemical rays are most abundant beyond the violet
end of the spectrum, and decrease through the violet, blue, and green, to
the yellow, where they cease. The rays of heat are in excess a little beyond the red end, and gradually decrease towards the violet end. Besides
these there are two insulated spots at a considerable distance from the red,
where the heat is a maximum. Were the rays of heat visible, they would
exhibit differences as distinct as the coloured rays, so varied are their pro.
perties according to their position in the spectrum. There are also pecun
liar rays which produce phosphorescence, others whose properties are not
quite made out, and probably many undiscovered influences; for time has
not yet fully revealed the sublimity of that creation, when God said, "Let
there be light- and there was light."