The object of the following remarks is to endeavour to give to the young gardener some insight into the nature of the subject in pursuit of which he has cast his lot, and is not intended in any way to supersede the necessity for his learning the art of horticulture from those who are capable and willing to take the trouble of teaching him, nor to lead him to neglect the study of Botany in all its branches in any of the advanced text-books on that science.

A practical acquaintance with some of the elements entering into the composition of plants - some knowledge of the climate, soil, and situation of the countries in which they grow, and of the diseases to which they are liable, or predisposed, and the causes thereof, and the remedies to be made use of, and of the hosts of destructive insects which feed on them, and of the methods for removing them, and of some of the elementary Laws of Physics - will materially assist the gardener in acquiring a sound knowledge of the practice of horticulture.

Some of the elements which are known to enter into and form part of the structure of plants are carbon, hydrogen, oxygen and nitrogen, sulphur, phosphorus, and chlorine, which, in combination with sodium, forms chloride of sodium (sea salt); and in some form or other plants derive nutriment from chlorine, iodine, bromine, and silicon, which, in the form of flint or silex, enters largely into plant structure, although more so in some plants than in others, and in combination with oxygen and metals it forms a class of minerals called Silicates.

Other of the elements are of the metallic class, such as calcium, in its carbonate state as limestone constituting vast mountains, and as chalk, enormous beds, which are decomposed by the action of the atmosphere and by rain-water, which, by-the-by, contains carbon dioxide (carbonic acid): this limestone or chalk when burnt in a kiln becomes calcium monoxide or quick lime. Aluminium in the state of an oxide of aluminium is clay, which is felspar weathered or exposed to the action of the atmosphere until disintegrated, and felspar is a double silicate of aluminium and potassium. Other of the elements are magnesium, potassium, and sodium, the two latter not identical in their operation, the one not supplying the place of the other in plant structure; and in combination with oxygen are the alkalis potassa and soda. The former (potassium) enters largely into the composition of land plants as a chloride, and is obtained by them from soils produced by disintegration of granite rocks. The presence of potassium in plants is easily proved by burning them, but the ashes, popularly called Potashes, will not contain the element in the state in which it was in the plant during its life, but in the state of a carbonate; it is said to enable the plant to prepare and form starch.

The latter (sodium) enters largely into the composition of marine plants.

Other elements in the composition of plants are iron, supposed to be necessary to the formation of chlorophyll, copper, manganese, and lithium, which last occurs in the ashes of the Grape Vine, Tea, Coffee, and Tobacco plants, in the milk of cows which have fed on plants growing in soils containing lithium, and in moor water, and is one of the most widely distributed elements.

Carbon, hydrogen, oxygen, and nitrogen have been called the organic elements or organogens; sulphur and phosphorus have been called pseudo-organic elements; and calcium, aluminium, magnesium, potassium, sodium, iodine, bromine, silicon, iron, copper, manganese, and lithium, have been called the inorganic elements. The distinction is not a desirable one, to say the least, since, whether any of them are essential to plant life, and others only partially and not universally present in plant structure, it seems apparent that, as soon as any plant has utilised any one of the so-called inorganic elements, it has then formed as much a part of the organic structure of the plant as carbon, hydrogen, etc. The only way out of the difficulty is to avoid the use of the words organic and pseudo-organic as inapplicable to the above elements in reference to plant life.

Some knowledge of the atmosphere by which our world is surrounded will soon be found necessary. The atmosphere or air is composed of the gases nitrogen, oxygen, aqueous vapour or vapour of water (produced by evaporation from water in some of its many forms, and its amount therefore varying with the temperature), and carbon dioxide (carbonic acid) and ammonia, which is composed of nitrogen and hydrogen. The vapour of water is the lightest of all gases, except hydrogen and ammonia, is invisible, is largely absorbed by plants, but on its condensation by cooling is visible as a cloud, mist, dew, rain, snow, or water, and to the latter forms of it we are indebted for the supply of water to our springs and rivers. It is to the vapour of water that we are indebted for the beautiful appearance of the atmosphere, which by preventing the too rapid radiation of heat from the earth, affords as it were clothing and protection to vegetable life.

A knowledge of the attributes of vapour of water or aqueous vapour will enable the gardener to understand and overcome many difficulties when he works in the plant houses, where he will soon see the effects of aqueous vapour evaporating from the open water tank, or from pools of water purposely thrown down, in its condensation on the cool glass, dripping down, and often spoiling the foliage of plants, especially of orchids; or in a vinery filled with late Grapes, if the lights are left open in dull moist weather, the aqueous vapour of the atmosphere will enter and condense on the cold berries of the Grapes and lay the foundation for many a rotten berry.

A knowledge of some of the laws of heat will be very serviceable; without heat the sap cannot rise in the plant cells, nor the seed germinate. To the expansion by heat of all bodies (whether solid, liquid, or gaseous) the gardener is indebted for the thermometer, which, by the rising or expansion of the mercury in the tube, shows on the face of it the degree of heat, whether in the open air, on the surface of the earth, or in the plant house, or even in the hot bed formed of stable-manure (if a thermometer specially constructed for that purpose is plunged into it), a great advantage by-the-by over the old fashioned way of leaving a stick plunged in the hot bed and pulling it out occasionally, and by the personal contact with the hand guessing whether the heat called bottom-heat in the hot bed was sufficient. Again, to the laws of the expansion by heat as applied to fluids, the gardener is indebted for the power of heating with the hot-water apparatus, as it is often called, the houses placed under his care; and to the pressure of the weight of the atmosphere the gardener is indebted for the barometer and the common pump.

To return to that part of our remarks in which carbon dioxide (carbonic acid) was mentioned as a constituent of our atmosphere. It is the great material with which the plant builds itself; most of the carbon in the plant is formed from carbon dioxide. It forms only a small portion of the lower part of our atmosphere; but owing to the law of diffusion of gases and other causes the quantity is never lessened, notwithstanding the continual abstraction of it by millions of hungry leaves. It is itself supplied to the atmosphere from active volcanoes, in large quantities, from fissures or cracks in the earth, from the breath and exhalations of animals and fishes, from burning substances containing carbon, from the decomposition of substances containing carbon dioxide, such as chalk or limestone (both of which are animal formations), and from rain and sea-water.