a way that they were supplied with all the elements. occurring in plants, with the exception of the one element under investigation. If the plant grew to maturity the element which was missing was deemed nonessential. If, on the other hand, the plant failed to develop, that particular element was considered to be essential.

The numerous experiments of this kind which have been carried on show that of the ash constituents potash, lime, phosphoric acid, magnesia, iron and sulphuric acid are absolutely essential to plant growth. Toward soda, chlorine and silica plants seem to be indifferent, as they can grow to maturity in the absence of these substances. For this reason it is generally conceded that only ten of the thirteen elements found in the plant are essential to growth, soda, chlorine and silica being thought non-essential. Accepting this view and referring again to the table on page II it is seen that 1,000 pounds of corn plant contain only 9 pounds of essential mineral matter or about 0.9 per cent. Attention is called to the fact that these experiments extended over only one generation, and that it is possible that an attempt to grow the crop through successive generations in a soil devoid of soda, chlorine or silical might show different results.

One Element can not be Substituted for Another. -The experiments mentioned above have shown, not only that certain chemical elements are necessary to plant growth, but also, that it is not possible to replace these essential elements even by others which are similar in chemical properties. In the chemical laboratory, for example, it is found that soda and potash are very

much alike in their action, and one may be used in place of the other in many operations. It would be a good thing for agriculture if soda could be substituted for potash as a plant food, as compounds of sodium are very inexpensive compared with potash compounds.

Spinach: a full ration soda; b full ration potash

One element of plant food cannot take the place of another in promoting plant growth. The pile on the right shows spinach grown with complete fertilizer. The pile on the left received the same fertilizer with potash replaced by soda.

This point has been thoroughly investigated, and it has been demonstrated that soda can not take the place of potash as a fertilizer. As a definite amount of each of these elements is required for a certain yield, and none of the elements can be replaced by another, it seems to follow that the crop produced will be limited. by the quantity of the essential element present in least proportion, compared with the requirements of the

crop.

potash sufficient for only half an average crop, no more than this can be produced no matter how much of the other forms of plant food is present.

How the Mineral Matter Enters the Plant.—It seems evident that the mineral matter must be taken up in some way by the roots. All are familiar with the fact that the soil is not a solid mass but consists of small particles, or "grains," with air spaces between, these spaces in the surface foot amounting to fully half the bulk of the soil. These grains vary in size according to the character of the soil, being very fine in clay, and comparatively coarse in sandy soils. The roots of the plant push down between these soil grains, branching more or less, and spreading throughout the soil. Surrounding the growing tip of the root are great numbers of fine root hairs that work their way in between and around the small soil grains, adher

ing closely to them and Root-hairs on wheat when very young

covering an immense amount of surface. It is on these root hairs that the plant is dependent

and four weeks later. All the water and food from the soil enter the plant through the root-hairs. Note how closely the root-hairs adhere to the soil particles. (After Sachs)

for the absorption of its water and mineral food.

It was

once thought that plants actually took in the very small solid particles of soil, and that the purpose of cultivation is to render the particles minute enough for the plant to absorb. It is now known that no food can enter the plant unless it is in solution. Each soil grain is surrounded by a film of water, and this water contains dissolved in it tiny quantities of the mineral ingredients of the soil, including nitrogen in the form of nitrates. The root hairs absorb the moisture as it is required by the plant, and with it such mineral matter as it needs. Both water and the dissolved matter enter the plant by the process known as osmosis. Each element is absorbed independently of the others, and the plant can in a way refuse to absorb more of any one ingredient when it has all that is needed for its growth. This "selective power" of the plant (if it may be so called) is shown by the fact that two different kinds of crops grown on the same soil may differ greatly in their composition. The ratio between the chemical elements found in them may be entirely different in the two crops and may be, in a great measure, independent of the ratio existing between these elements in the soil water.

Soil Solutions Very Dilute.-The amount of mineral matter in the soil water is very minute. In the second chapter attention was called to the fact that at least 300 pounds of water must pass through the plant to produce one pound of dry matter. The fact that the soil water contains mere traces of plant food probably accounts, in some measure, for the immense quantity of water used by the plant, as it must absorb. this water to obtain the food it requires. The plant is

not entirely dependent upon the mineral matter actually dissolved in the soil water for its supply of food. The roots have the power of secreting an acid substance that has a solvent action on that part of the soil which is insoluble in pure water. This is shown by the root tracings often seen on pieces of limestone in the soil. It may be shown by growing a plant in a small quantity of soil placed on a piece of marble. If the marble is examined after a time the outlines of the roots can be seen distinctly where the acid substance has cut into its surface. How great a factor in obtaining food this property of the plant is can not be stated at present on account of our limited knowledge of the subject.

Function of the Different Food Elements.-Now that the source of the different elements required by the plant has been briefly discussed, it is desirable to have explained the special function in the vital processes of the plant performed by each of these substances. Unfortunately but little is known in regard to this subject, for up to the present time it has almost defied investigation. Carbon, oxygen and hydrogen are found in all the organic compounds of the plant and form about 981⁄2 per cent of the green corn crop. Nitrogen is a constituent of proteids and is necessary to their formation. Sulphur is found in some of the proteids but its special function is not known. Phosphoric acid is supposed to be in some way connected with the transportation of the proteids from one part of the plant to another. Potash is thought to be necessary to the conversion of starch into sugar and, consequently, to its removal from the leaves to other parts