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.