absence of light this action is temporarily suspended, and slightly reversed.

By means of these breathing pores, not only carbonic acid, but other gaseous substances which are present in the atmosphere, may be introduced into the plant. Hence we find the leaves throwing off large quantities of moisture in addition to the oxygen, and in the exchange supplying the plant largely with materials for growth. Thus we have two distinct agencies whereby plant-food is received, each discharging different functions, and each remarkable for their very minute formation,-the millions of little filters through which the water passes from the soil, charged with its dissolved gaseous and mineral matter as food for the plant;-whilst myriads of equally minute organs are aiding the distribution and use of the nourishment, by exhaling moisture from the leaves, and helping to perfect the conditions of vegetable growth. The agency by which plants. receive their food is singularly beautiful and complete, and in every way well adapted for making the best use of any nourishment which may be supplied, whilst at the same time exercising a guardian care over its admission to the plant. If such be the characters of these very trusty "gatekeepers," which guard the precincts of the region of vegetable growth, well may we long to know more of the mysterious changes which take place within the plant, where plant-food becomes endowed with the attributes of life, as it becomes more and more perfectly transformed into a living organism.

PART THE SECOND.

CHAPTER XX.

THE fertility of any soil is shown by the successful growth of those crops which are suitable for the climate of the district in which such a soil may be situated. The climate and management of the land being favourable, the growth of our cultivated crops give distinct evidence as to the productive powers of a soil. All three conditions are necessary for luxuriant growth. If the climate and the course of tillage operations are unfavourable, the fertility of the soil is not enough to secure the production of large and remunerative crops. On the other hand, a favourable climate and good husbandry, cannot make a soil fertile which does not contain the necessary plant-food. In speaking, therefore, of the influence of plant-food on the productive powers of a soil, it must be regarded as one essential condition of a threefold alliance. First, we require the necessary supply of plant-food, which may be shown by a knowledge of the chemical composition of the soil. Secondly,

we require good tillage operations, which shall secure the proper mechanical condition of the soil, which must be rightly made use of by a man possessing farm experience. Thirdly, we require favourable conditions of climate. These constitute a threefold alliance, and for the successful cultivation of the soil each of them must contribute its share to the desired success. Hence a proper supply of plantfood is absolutely necessary, but this alone will not secure successful results.

Under a judicious course of farm management the plant-food in a soil will receive careful consideration on the part of the farmer; for unless the crop can obtain the materials it requires for a full and luxuriant growth, the produce must be limited by any deficiency in the supply. The requirements of the crop must of necessity embody a supply of all the materials which are contained in a luxuriant

yield of that crop. In this respect the productions of vegetable life in a great measure correspond with the structures built by the skill of man; the materials existing in the completed work show the smallest quantities needed for the prosecution of the work. These at least are necessary. Hence it is that the mineral constituents found in the ashes of a plant, show the minimum requirements of this description of food. We have no such definite guidance in the demands made by plants for the building up of the organic portions of their structure. It is true that by analysis we can estimate with some degree of accuracy the quantities of carbon, hydrogen,

oxygen, and nitrogen they contain, but we cannot guide our proceedings by these data. So far as regards the three first-named substances, these are abundantly supplied from natural sources. The carbonic acid present in the air enables the leaves of plants to draw from the atmosphere abundant supplies of carbon; but there are other duties to be discharged in the vegetable economy which render a supply of carbonic acid necessary in the soil. The supplies of hydrogen and oxygen need little special care, for the water (which is composed of these two gases) entering into the plant becomes largely decomposed, and yields both of these gases. In the case of nitrogen the circumstances are altogether different, and its judicious supply is a subject which demands the greatest care, and will be again referred to. For the moment we may, therefore, consider that we have especially to secure in the plant-food of any crop-first, the mineral matter built up in a good yield of that particular plant; and secondly, the necessary supply of nitrogen. In the following table a statement is given showing what mineral matters moderate crops of wheat, beans, turnips, and clover contain. It may be fairly assumed that still larger crops will make proportionately larger accumulations of these mineral matters, but this admits of ready calculation according to the circumstances of each case.

The plant-food necessary for these crops, it will be seen from the above, includes a large number of different bodies, and the quantities used of each of these substances varies considerably. The question has arisen in the minds of some whether these bodies are all required, or whether any of them can be replaced by others in the group. It may be safely stated that for securing a luxuriant growth, each and all of these bodies are necessary. Substitution undoubtedly takes place within certain narrow limits, but the action of such deputies is inconsistent with a full maintenance, either of an abundant yield, constitutional strength, or reproductive power. For all practical purposes any substitution of one kind of plant-food for another,