TRUE AND FALSE DEW
Ever since men could observe and think, they have admired the diamond
globules sparkling in the rising sun. These "dew-drops" were considered to
be shed from the bosom of the morn into the blooming flowers and rich
grass-leaves. Ballantine's beautiful song of Providential care tells us
that "Ilka blade o' grass keps it's ain drap o' dew."
But, alas! we have to bid "good-bye" to the appellation "dew-drop." What
was popularly and poetically called dew _is not dew at all_. Then what is
it?
On what we have been accustomed to call a "dewy" night, after the
brilliant summer sun has set, and the stars begin to peep out of the
almost cloudless sky, let us take a look at the produce of our vegetable
garden. On the broccoli are found glistening drops; but on the peas,
growing next them, we find nothing.
A closer examination shows us that the moisture on the plants is not
arranged as would be expected from the ordinary laws of radiation and
condensation. There is no generally filmy appearance over the leaves; the
moisture is collected in little drops placed at short distances apart,
along the edges of the leaves all round.
Now place a lighted lantern below one of the blades of the broccoli, and a
revelation will be made. The brilliant diamond-drops that fringe the edge
of the blade are all placed at the points where the nearly colourless
veins of the blade come to the outer edge. The drops are not dew at all,
but the exudation of the healthy plant, which has been conveyed up these
veins by strong root-pressure.
The fact is that the root acts as a kind of force-pump, and keeps up a
constant pressure inside the tissues of the plant. One of the simplest
experiments suggested by Dr. Aitken is to lift a single grass-plant, with
a clod of moist earth attached to it, and place it on a plate with an
inverted tumbler over it. In about an hour, drops will begin to exude, and
the tip of nearly every blade will be found to be studded with a
diamond-like drop.
Next substitute water-pressure. Remove a blade of broccoli and connect it
by means of an india-rubber tube with a head of water of about forty
inches. Place a glass receiver over it, so as to check evaporation, and
leave it for an hour. The plant will be found to have excreted water
freely, some parts of the leaves being quite wet, while drops are
collected at the places where they appeared at night.
If the water pressed into the leaf is coloured with aniline blue, the
drops when they first appear are colourless; but before they grow to any
size, the blue appears, showing that little water was held in the veins.
The whole leaf soon gets coloured of a fine deep blue-green, like that
seen when vegetation is rank; this shows that the injected liquid has
penetrated through the whole leaf.
Again, the surfaces of the leaves of these drop-exuding plants never seem
to be wetted by the water. It is because of the rejection of water by the
leaf-surface that the exuded moisture from the veins remains as a drop.
These observations and experiments establish the fact that the drops which
first make their appearance on grass on dewy nights are not dew-drops at
all, but the exuded watery juices of the plants.
If now we look at dead leaves we shall find a difference of formation of
the moisture on a dewy night: the moisture is spread equally over, where
equally exposed. The moisture exuded by the healthy grass is always found
at a _point_ situated near the tip of the blade, forming a drop of some
size; but the true dew collects later on _evenly_ all over the blade. The
false dew forms a large glistening diamond-drop, whereas the true dew
coats the blade with a fine pearly lustre. Brilliant globules are produced
by the vital action of the plant, especially beautiful when the deep-red
setting sun makes them glisten, all a-tremble, with gold light; while an
infinite number of minute but shining opal-like particles of moisture
bedecks the blade-surfaces, in the form of the gentle dew--
"Like that which kept the heart of Eden green
Before the useful trouble of the rain."