Mr. Blodgett gives an excellent table of the temperature of the principal wine districts of this country. (See Table B. p. 39.) 6. It has been shown that heat is a great stimulus to vegetable life. Cold, on the other hand, diminishes the activity of plants. At certain seasons of the year a sudden fall in the temperature produces in some kinds of vegetation disease and death. If a frost occurs late in the spring, when all the organs of the tree are performing their work with great rapidity, the leaves and all those soft parts which are filled with fluid become black and stained, and lose their vitality. The green portions of a tree are made up of cells, whose membraneous walls are very thin. On the contrary, those of the wood are very thick; but this varies according to their age. When the juice contained in the former, or young cells, has become frozen, it ex pands the slight walls until they are unable longer to withstand the pressure, and burst. When their temperature is restored, the sap, of course, runs out among the tissues of the wood, and its nutritive action is lost. As the injury done by freezing is thus not made manifest until the heat is restored, it has often been thought that it was in the thawing that the leaf was destroyed. It is no doubt true that by shading from the sun, or by washing with water, these organs are sometimes enabled to bear the immense damage which they have sustained, and to recuperate their energies, through the action of those parts which have not been injured; but the harm is accomplished before it becomes evident to the casual observer. A low degree of temperature, even though it be not at the freezing-point, is injurious to plants, as it causes stagnation of the sap, in which case fungi are liable to attack their tissue. Severe cold during the winter is generally not productive of injury, because these membraneous walls have then become thickened by layers of woody fibre, and have greater resistive force; the flow of sap also being then so much less, the danger to the tree from its expansion by freezing is lessened. Steady cold weather during the winter is beneficial to vegetation. Many tender plants will endure our climate if protected from the sun, which defends them from these variations in temperature. This is the principle which should be followed in covering all half-hardy trees or shrubs. It is not necessary to shield them from the cold, which one would judge was the object from the immense bundles of straw with which many such plants are surrounded, but to shade them so as to prevent those sudden changes, from heat to extreme cold, which destroy the tissue. Mild winters are those most destructive to vegetation, as they are generally characterized by great reverses of temperature. A writer says:1 "We speak of one year as warmer or colder than another; but it is a wonderful example of unchanging law, that they seldom differ materially in the mean temperature of the year. That of London is fifty degrees, four minutes; and, however hot or cold the seasons were, it did not cause the average of the year to vary more than one-half a degree; and this was probably owing to the imperfection of our implements." A warm summer is very beneficial to vegetation, because, as we have already seen, a constant high temperature at that time stimulates their vital energies. But as the mean of every year is nearly the same, in order to have a warm summer the preceding winter must have been cold. Sometimes, however, the heat following a cold winter seems to be misplaced, either by an unusually warm spring or autumn, which produces the mean, even though the 1 Loudon's Gardeners' Magazine, Vol. XVII. p. 147. temperature of the summer should be moderate. Thus the seasons following a cold winter are the golden ones for the fruit-grower. 7. The effect of electricity may be considered in treating of the causes of American pear-blight. 8. Light is another essential requirement of vegetation. It has a great influence in maturing the wood of the plant. In places where it is absent the foliage becomes sickly, and a poor, unripe growth is the consequence. It affects not only the growth of the tree, but also the quantity and quality of the fruit. A writer1 in the Gardeners' Chronicle says: "I send you a few peaches, taken from a tree which was brought to this garden as a nursery-plant in 1832; the following winter it was planted where it now stands, the wall and border being both new. For the first ten years I do not recollect that it ever bore a fruit, owing to a large sycamore which overshadowed the wall where it stood. The tree being an object visible from various parts of the premises, my master felt the greatest reluctance to take it down; but about ten years ago he consented to remove it, and since that time the peach tree, which had never before that carried a single fruit, has rewarded us with a fine crop every year. The number this year upon the tree was thirty dozens; 1 John Povey, Thorneycroft Hall, England. |