This section is from the book "The Gardener's Monthly And Horticulturist V23", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
In the May number of the Monthly " Irrigation " makes some remarks on " watering plants by leaf absorption." There are few persons now who will disagree with him in regard to the conclusions reached by Boussingault, that plants absorb little or no moisture through their leaves from the atmosphere. But, is this assumed as a scientific reason for not giving plants a moist air to grow in? It may look well enough as theory, but the practical plant grower finds it necessary, yes, absolutely necessary, to surround certain classes of plants with a proper medium of moisture to keep them in healthy condition. We syringe plants, not with the intention of feeding them by watering the surface of the leaves, but for the cleansing which the operation affords and to stay the rapid waste of the fluids of the plants from within, a waste which increases with increase of temperature. The babe's mother or nurse washes or bathes her precious charge not in milk, but in water; not to feed it you will grant, but to cleanse it, and the operation is generally conceded to be healthful to the babe so treated.
It is only the excess of moisture that does the injury, and your correspondent gives a striking example of this in relating the experiment of his geranium and pomegranate, not only a drop of water falling often in one place, but a fragment of a fallen flower or any other decaying substance remaining on the leaf of a geranium that absorbs moisture would have the same effect - to produce decay.
The propagator who has to handle thousands of plants grown mostly from cuttings, knows well the necessity for the moisture he applies through his sprinkling pot; and also that the result of withholding that moisture would in a short time be a lot of dead leaves instead of growing plants.
Irrigatum is, however, right in his assertion that the under side of the leaves of plants transpire the most. This is true even of the leaves of unrooted cuttings. In propagating green cuttings, particularly those having large leaves, the upper surface may become apparently quite dry without injury to the cutting, but let the under surface become dry and remain so for an undue time, the death of the leaf and consequent loss of the cutting is certain. This dryness is avoided by sprinkling the upper surface of the leaves and all their surroundings, thus saving or storing in the stem and leaves of the cuttings the fluids which nature has placed there for the elaboration of the future plant.
In all cases a proper adjustment or alternation of moisture and dryness both of roots and leaves is necessary to keep plants in a healthy, growing condition. This every one soon finds out who by practice becomes accustomed to the general requirements of plants.
There are, indeed, many establishments where this general application of moisture to the leaves of growing plants is carried to excess. In such places there is always much loss of plants by "damping off," as it is called, and also considerable difficulty in raising seedlings. The excessive moisture produces fungus, which literally eats up everything of delicate nature. The too free use of the hose and force pump, particularly in winter and the colder days of the early spring months, is one fruitful source of this kind of mischief. In the proper season water power and hose are of incalculable service in plant growing on an extensive scale. Nevertheless it is an agent that must be used with judgment in order to reap the full benefit of it.