These may be considered separately: 1st—It may be assumed, and has been already well established by botanists, that every plant has its own peculiar constitution, adapting it to certain atmospheric conditions, and that for its healthful and successful culture, these must be understood and adhered to, within comparatively narrow limits. Tropical plants, as is well known, cannot be cultivated beyond their natural limits, except under circumstances where their natural conditions are nearly imitated by the gardener; and even in our stoves and hot-houses, these plants do not compare in vigor with their fellows that luxuriate in the hot and steaming atmosphere of the tropics, under the stronger light of such a clime as is natural to them. On the contrary, the plants of northern latitudes will not grow and produce seeds where temperature is too elevated. Those from a humid atmosphere suffer in an arid clime, and those which thrive in dry sandy regions suffer equally when introduced into a humid atmosphere.
Thus we find, that where there is too much moisture for some of our cultivated plants, they are inclined to be too succulent, and this very excess may produce a dropsical condition that is really a state of disease. Thus we suffer in a loss of fruit, which will fall badly before its period of maturity, and that which remains its full time is found to be thin and watery, deficient in the high spicy aromatic flavor which is so highly appreciated by the connoisseur of these choice products. When, on the other hand, the arid character of the soil and climate prevail to an extent that is uncongenial to any particular fruit, we shall find that its growth is arrested, and that its highest qualities are not adequately developed: this is frequently observed in an unusually dry season—and in California, where irrigation is required to enable the orchardist to produce some of the succulent fruits, the most remarkable size and beauty have been attained, but we are told that it was often at the expense of the desired flavor that the same varieties acquire, under circumstances more advantageous to the development of their superior qualities.
So in many of our fruits, the successful results depend upon the hygrometric condition of the atmosphere, and Liebig suggests that a very prolific source of diseased action in plants, arises from the suppressed evaporation and transpiration consequent upon such atmospheric conditions.
Too much moisture prevailing at the time of the blossoming of our fruits, especially moisture precipitated in the form of rain during this period, is sometimes disastrous to our crops, both of cereals and of orchard fruits. Continuous showers prevent the development of the pollen-grains, and their transfer to the stigmas of the blossoms, so that the fruit does not set well. Fortunately this does not often occur in our glorious climate, which is so highly favored by an abundance of light and sunshine, which are the great and essential stimuli of the higher orders of plants. The loss of our fruit crops in some parts of the Ohio Valley in the years 1862, 1865, and 1866, was fairly attributed to this cause.
We must not overlook the unhealthy influences produced by an excess of moisture in the earth. Many plants that naturally delight in a dry porous soil, become weak, unfruitful, or even seriously diseased when they are planted in low wet grounds, or upon such as are underlaid by a very tenacious sub-soil, while an opposite condition is equally unfavorably to those that are naturally more aquatic in their tastes and habits. In the former case we learn to avoid such soils and situations, unless we are able to change their character in this respect by thorough under-draining, which will completely remove the evil, and the remedy becomes merely a question of expense.
A certain amount of temperature may be assumed as requisite to every plant, or rather it may be affirmed that some plants cannot exist and thrive except within a certain range, and it has been asserted that each class of plants requires a mean temperature for the year that shall not vary many degrees: the range of this variation has perhaps never been satisfactorily ascertained. But it is well known, that both heat and frost act injuriously upon vegetation. Mr. Lindley tells us that "the extreme limits of temperature which vegetables are capable of bearing, without destruction of their vitality, have not been determined with precision." When the temperature is maintained at a higher point than is natural, the plant is excited to undue activity of growth; but this is attended with an enfeebled condition, often seen in badly managed green-houses. Mr. Knight found that certain plants were rendered abortive by the production of male flowers only, when exposed to too great heat, and by an opposite treatment, when subjected to a low temperature for a long time, others produced only female flowers. In some plants a high degree of heat, with moisture, results in the production of leaves only, and Humboldt found that wheat was grown about Xalapa, Mexico, as a fodder plant, because it produced an abundance of grass, but did not form ears nor grain.
A diminished temperature, on the contrary, removes the stimulus of growth, and leads to the suspension of all vital action in proportion to its reduction. At the freezing point it is probable that all such action ceases, though in this regard there is great difference among plants; the mosses and lichens will flourish, and the Chickweed will vegetate and blossom at a temperature very little above freezing. The access of frost, after vegetation has somewhat advanced, often proves very disastrous, and we not unfrequently lose our crops of fruit by such an occurrence during the period of blossoming, or even afterward.
Some plants in a dormant condition, will endure uninjured a great depression of temperature, while others will be destroyed by the slightest approach of frost. According to De Candolle, this may depend upon the greater or less amount of water they contain, upon the greater or less viscidity of their fluids, or the rapidity with which these fluids circulate. Those with larger cells he thinks most easily injured by frost, and those which contain a great deal of air are able to resist it best. The freezing point will vary according to the quality of the sap, for we know that different vegetable juices congeal at different temperatures. The manner in which cold acts upon plants depends upon their physical structure. Lindley says, freezing is attended with the following effects:—The fluids contained within the cells of tissue are congealed and expanded—this produces a laceration of the cell-walls, and impairs excitability by the unnatural extension to which the cells are subjected; the air is expelled from the air-vessels and introduced into parts naturally intended to contain only fluid; the green coloring matter and other secretions are decomposed, and the vital fluid or latex is destroyed, and the action of its vessels is paralyzed. The interior of the tubes, in which fluid is conveyed, is obstructed by a thickening of their sides. So we have as a result, both mechanical, chemical, and vital changes.[16]
Our hardy fruit trees are woody perennials that hybernate during the winter. Yet we find that even these suffer upon some occasions from a great depression of temperature; it has been asserted that a certain degree of cold would inevitably destroy the blossom buds at least, and we often find that the bark is burst off from the wood, and in some instances the wood itself is so injured as to suffer from a kind of decomposition, and to become affected with a change generally known as the dry rot, losing its elasticity and hardness, and acquiring a whitish color, which is supposed may arise from the introduction of the mycelia of fungous growths. Now it is believed that these injuries do not arise so directly from the degree of cold to which the tree has been exposed, as to the condition of its circulation at the time of the exposure. If the sap have been excited by mild or warm weather, as is so apt to be the case in our changeable climate, the sudden depression of temperature will produce disastrous effects, even when the cold has not been very severe. This is manifested by the bursting of the bark in young trees in the early part of winter, while they are yet holding their leaves, and of course having a circulation somewhat active. Hence the importance, now very well understood by our nurserymen, of checking the growth of young trees in time to have their terminal buds thoroughly ripened before the approach of frost. This, to a certain extent, is subject to our control; but we cannot foresee the character of the seasons upon which the safety of our orchard trees will, in a great measure, depend, and they are less easily managed. When the autumn is dry, and continued late into winter, as sometimes happens, we see a perfect ripening of the wood, with a great development of blossom buds, and then we may confidently calculate upon the safety of our fruits, provided they be not exposed to a warm period at mid-winter, that shall excite some activity in their circulation, which would suffer terribly from any sudden and great depression of temperature such as frequently occurs, carrying the mercury from summer heat to a point below zero, in a few hours. Such a change has amounted to 68 degrees in nine hours.[17]
The influence exerted by the soil upon the healthiness or unhealthiness of our trees has already been alluded to incidentally, but it is an important subject of inquiry whether this may arise from a redundancy or a paucity of some particular ingredients necessary to sustain the plants we desire to cultivate. Liebig has pointed out how chemistry may be brought to our assistance in solving such a question. As all the inorganic elements found in a tree and its fruits, must have been derived from the soil in which it grew, he suggested that the ashes of the plant would show us exactly what it needed, and then an examination of the soils would inform us whether they contained all the necessary elements, and in the right proportion. Hence arose the doctrine and the practice of applying special manures, which has been so fashionable in our day. Though there be many doubters as to the efficacy of such investigations and practices, most sensible and enlightened agriculturists admit the truths which Liebig has propounded.