3. Severe winds are often followed by most disastrous results. Trees are broken down, and frequently torn out of the ground by the roots, when exposed to its full power upon an unprotected plain. This is one of the most serious difficulties of fruitgrowing upon some of the prairie lands of the West, but one which is easily overcome. Belts of pines, or other evergreens, planted at the most exposed point, will in a great measure prevent injury. In more hilly districts the strong winds may not be from the west; currents draw through the valleys, and the shields must be placed at the point of exposure. In early summer, when the tree has just started its young growth, a more than ordinary wind will so whip these young shoots against each other as to change the whole appearance of the orchard in a very short time: when, a few hours before, each tree was beautifully green, now all of the young leaves upon the ends of the shoots have become bruised, black, and dead. The tree will soon recover from the injury; but the rapid elaboration of the sap has been checked, and much growth lost.

In the vicinity of Boston there prevails annually a severe northwest wind, about the twentieth of September, which strips from the trees hundreds of bushels of fruit. This injury might be prevented, in a great measure, by proper shelter.

Positions where the air is stagnant should be also carefully avoided. The atmosphere contains a cer

tain proportion of carbonic acid, which is the chief source of the carbon of plants, constituting the greater part of their wood. This is received by the plant not only through the roots, but by means of all the green portions of the tree. If the air was not in circulation, it can readily be imagined that this gas would be more or less exhausted in that part of the atmosphere which immediately surrounds the plant, and that the tissue would consequently be pithy and soft. Therefore we can discern the use, in the great system of Nature, of those gentle breezes which so lightly rustle the foliage, and supply to the most minute leaf its proper share of this great element of life. The stagnation of the air, together with electrical influences, we shall discover, when we investigate the diseases of fruit-bearing plants, to be the probable causes of the American pear-blight.

4. Aqueous vapor. Not only does the air contain carbonic acid with hydrogen and oxygen, but aqueous vapor, which affects plants powerfully. The first three constituents form the primary elements of the bodies of both animals and plants; these, by their death and decay, restore the gases to their original condition; but the rapidity of these phenomena is regulated, in a great measure, by the presence of watery vapor. Its quantity varies with the locality and the season of the year. Its absence would cause

sterility. If the atmosphere in the African deserts could be saturated with moisture, they would become as verdant as any other portion of the earth.

The amount of moisture, or water, which the atmosphere is capable of containing, to become fully charged, varies with the degree of temperature. If it is eighty degrees Fahrenheit, it will contain ten grains to the cubic foot; if it be twenty degrees Fahrenheit, the amount of vapor would be little more than one or one and a half grains. Such being the case, a sudden fall of the temperature would result in the precipitation of the superfluous moisture. The diurnal depression of the thermometer is followed by dew, in consequence; or, when cold enough to freeze, by hoar-frost, which is merely frozen dew. Every one is aware of the clearness of the air in the winter, because its low degree of temperature does not allow it to absorb much aqueous vapor. When the air becomes heated in the spring, evaporation goes on very rapidly to supply the deficiency in the atmosphere, and thus nature dries up the soil.

The vapors contained in the atmosphere, and its reluctance to part with heat, are among the reasons why, in our latitude, the temperature does not sink to the freezing-point every night. Late frosts in the spring are not so injurious to vegetation on the coast as inland, because, when the sun is up, and the temperature begins to rise, a fine mist immediately ascends to supply the want of the atmosphere,

and thus shades the plant from the otherwise destructive rays of the sun.

5. Heat is an important stimulus to vegetable life. The functions of most plants cannot be performed without a certain degree of this agent. The more rapidly the tree is growing, the more is it dependent on the maintenance of a high temperature. The requisite degree, however, differs widely with the species, or even variety. Mr. Lymburn speaks of a plant of Marchantia which was growing in a hot spring on the island of Amsterdam, where the water was above the boiling-point; while, on the other hand, the curious Protococcus nivalis adorns the polar regions, where the frost scarcely gives way under the heat of midsummer: and yet this plant spreads over vast plains, and illumines them as if by crimson

snow.

A high degree of heat is generally favorable to the growth of fruit, if it be not accompanied with drought. By the management of artificial heat, gardeners make plants perform curious freaks. Sir Thomas A. Knight caused melons and cucumbers to produce all male blossoms in excessive heat, and all female in a low temperature. Heat stimulates evaporation from the leaves, and therefore excites the roots to absorb nourishment from the soil. With the aid of light it paints the colors upon the fruit, which show that the saccharine fermentation has been well performed.

LIMITS OF THE VINE.

37

It is often noticed that the sweetest fruits are nearest to the ground, because they obtain a greater amount of heat by its radiation from the earth. Mr. Murray, of England, proved that this was very dif ferent on the side of a hill from what it was in the valley. In one case the thermometer was thirty degrees higher on the inclined surface than in the plain. He states that upon the plains of Piedmont, in Italy, the vignerons are obliged to detach their vines from the poles, and cover them during the winter, to prevent injury; while on the acclivities which surround the city of Genoa, the pomegranate, the lemon, and the orange, flourish.

The principles of radiation were understood in very ancient times. In the land of Judea the vineyards and orchards were often planted upon terraces, to acquire additional heat.

In Europe they fixed the limits of the vine by the mean temperature of the summer months. The least mean degree required for the ripening of grapes suitably for wine is sixty-seven degrees. Boussingault thus states the effects of the temperature upon the quantity and quality of wine produced.1 (See Table A, next page.)

"In 1833 and 1837, the wines were scarcely drinkable. A summer whose mean temperature is below sixty-seven degrees will not produce valuable wine."

1 Blodgett's Climatology, p. 439.