The reason why : $b a careful collection of many hundreds of reasons for things which, though generally believed, are imperfectly understood. A book of condensed scientific knowledge for the million
John Stuart Mill · en
Because, when the ear is becoming ripe, the cold dew falling into the
ear, might _induce blight_; the ears therefore turn down to the earth,
and _receive warmth by radiation_.
1176. _Why have grasses, corn, canes, &c., joints, or knots in their
stalks?_
Because a long hollow stem would be liable to bend and break. But the
joints are so many points where the fibres are bound together, and the
structure _greatly strengthened_.
[Verse: "Then shall the earth yield her increase; and God, even our
own God, shall bless us."--PSALM XLVII.]
1177. _Why have the berries of the mistletoe a thick viscid juice?_
Because the mistletoe is a _parasitical_ plant, growing upon the bark
of other trees. It will not grow in the ground; its seeds are therefore
filled with an exceedingly sticky substance, which serves to attach
them to the bark of trees, to which the berries attach themselves at
once, by throwing out tough fibres; and the next year the plant grows.
[Illustration: Fig. 75.--THE MISTLETOE.]
1178. _How are the seeds of the mistletoe transferred from its own stem
to the bark of trees?_
Various birds, and particularly the _missel thrush_, feed upon the
berries. As the bird moves in pursuit of its food, the viscid berries
attach themselves to its feathers, and in this way the thrush is the
instrument which conveys the seed to the spot to which it adheres, and
from which the tree ultimately grows.
1179. _What is the circulation of the sap in plants?_
The circulation of the sap is the movement of the nutritive juices by
which the plant is sustained. There is a slow uninterrupted movement
of the sap from the root through the stems to the leaves, and downwards
from the leaves through the bark to the root.
[Verse: "For the sun is no sooner arisen with a burning heat, but it
withereth the grass, and the flower thereof falleth, and the grace of
the fashion of it perisheth: so also shall the rich man fade away in
his ways."--JAMES I.]
1180. _Why does the sap of plants thus ascend and descend?_
Because it _conveys upward_ from the ground some of the matter by which
the plant is to be nourished, and which must undergo digestion in the
leaves; and it _brings downward_ from the leaves the matters absorbed,
for the nourishment of the plant, and discharges through the root the
substances which the plant cannot use.
The movement of the sap is most active in the spring; but in the depths
of the winter it almost ceases.
There are other motions of the sap in plants, which are called
_special_, in distinction from the ascending and descending of
the sap, which is called _general_, or common to all plants. The
special movements of the sap are peculiar to certain plants, in
some of which a fluid, full of little green cells, is found to
have a rotatory motion; in other plants, a milky fluid is found to
move through particular tissues of the vegetable structure.
1181. _Why are the leaves of plants green?_