in one case, which might not follow in the case of another type of compound; but the relative efficiency as plant food of a given unit of potassium seems to be the same regardless of the nature of the compound in which it is present.

Calcium is an essential plant food element but its physiological use has not yet been definitely established. It seems to stimulate root-development, and certainly gives vigor and tone to the whole plant. It is commonly believed that calcium is in some way connected with the development of cell-wall material. It has been reported that the stems of grasses and cereal plants become stiffer in the presence of ample calcium, but this may be due to greater turgidity rather than to strengthened cell-walls. Calcium remains in the leaves or stem as the plant ripens, but it is not clear that this has anything to do with the stiffness or weakness of the stem, or straw, of the plant. Experiments with algæ have shown that in the absence of calcium salts mitotic cell division takes place, showing that the nucleus functions properly, but the formation of the new transverse cell-wall is retarded. This is the only direct evidence that has been reported that calcium has any connection with cell-wall formation.

Certain species of plants, notably many legumes, require such large amounts of calcium salts for their growth as to give to them the popular appellation of "lime-loving plants." Other plants, known as "calciphiles," while not actually showing abnormally large percentages of calcium in their ash, flourish best on soils rich in lime. On the other hand, certain other species, known as "calcifuges," will not grow on soils which are even moderately rich in lime; in what respect these differ in their vital processes from others which demand large amounts of calcium, or those which flourish on soils rich in lime, has not been determined, however.

The beneficial effect of alkaline calcium compounds in the soil, in correcting injurious acidity, in improving the texture of clay soils, and in promoting the proper conditions for bacterial growth, is well known; but this has no direct connection with the rôle of calcium as plant food. Furthermore, calcium salts in the soil have a powerful influence in overcoming the harmful, or toxic, effects of excessive amounts of soluble salts of magnesium, sodium, or potassium, in the so-called "alkali soils" (i.e., those which contain excessive amounts of water-soluble salts). The probable explanation for this fact is pointed out in a later paragraph of this chapter (see page 14); but this property of calcium probably has no connection with its physiological uses as plant food.

Magnesium, like phosphorus, is finally stored up mostly in the seeds, not remaining in the leaves and stems, as do calcium and potassium. This fact, together with other evidence obtained from experiments in growing plants in culture solutions containing varying amounts of this element, has led certain investigators to the conclusion that the rôle of magnesium is to aid in the transport of phosphorus, particularly from older to more rapidly growing parts of the plant. More recent investigations have shown, however, that magnesium has other roles which are probably more specific and more important than this one. It is now known that magnesium is a definite constituent of the chlorophyll molecule serving, as will be shown (see Chapter VIII), as the means of linkage between its essential component organic groups. Because of this fact, magnesium-starvation produces etiolated plants, which cannot function normally. Further, magnesium seems to be necessary for the formation of fats, apparently standing in a similar relation to fat-formation to that of potassium to carbohydrate-formation. This view is supported by the observations that when algæ are grown in magnesium-free solutions they contain no fat globules and that oily seeds are richer in magnesium than are those which store up starch as their reserve food material. Observers of the second of these phenomena have failed to note, however, that oily seeds are likewise richer in phosphorus than are starchy ones, and that the presence of larger proportions of magnesium in such seeds may, perhaps, be related to phosphorus-translocation rather than to fat-formation.

Whatever relation magnesium may have to fat-formation, or to the translocation of phosphorus, it is evident that these are rôles quite apart from its use as a constituent element in chlorophyll. As yet, no explanation of how it aids in these other synthetic processes has been advanced.

On the other hand, an excess of soluble magnesium salts in the soil produces definite toxic effects upon plants, magnesium compounds being known to be among the most destructive of the "alkali soil" salts. Calcium salts are remarkably efficient in overcoming these harmful effects of magnesium salts. On this account, a large amount of experimental study has been given to the question of the calcium-magnesium ratio in plants. Numerous analyses of plant ashes have established the fact that there is a fairly definite ratio of this kind, which ratio, however, varies with the species of plant and is not correlated with the ratio of these elements present in the soil on which the plant grows, as was formerly believed. Cereal plants, as a rule, contain approximately twice as much lime as magnesia; while leafy plants (tobacco, cabbage, etc.) usually contain about four times as much calcium oxide as magnesium oxide.

Iron is essential to chlorophyll-formation. It is not a constituent of the chlorophyll molecule, as is magnesium; but in the absence of iron from the culture solution, a plant fails to produce chlorophyll and a green plant which is deprived of a supply of iron rapidly becomes etiolated. The way in which iron is related to chlorophyll-formation is not known.

Iron is taken from the soil by plants in the smallest proportions of any of the essential elements. Only soluble ferric compounds seem to serve as a suitable source of supply of the element; ferrous compounds being usually highly toxic to plants.

Sulfur is an essential element of plant food. The amounts required by plants were supposed, until recently, to be relatively small. This was due to the fact that earlier studies took account only of the sulfur which, on analysis, appeared as sulfates in the ash. Improved methods of analysis, which insure that the sulfur which is present in the plant tissue in organic combinations is oxidized under such conditions that it is not lost by volatilization during the combustion of the material, have shown that the total sulfur which is present in many plants approaches the quantity of phosphorus which is present in the same tissue. Furthermore, recent field and pot experiments have shown that at least a considerable part of the beneficial effects of many fertilizers, which has previously been attributed to the calcium, potassium, or phosphorus which they contain, is actually due to the sulfur present as sulfates in the fertilizers used.

Sulfur occurs in the organic compounds of plants, associated with phosphorus. It seems probable that its physiological uses are similar to those of the latter element; but there is as yet no experimental evidence to establish its exact rôle in the economy of plant growth. It appears to be needed in largest proportion by plants which contain high percentages of nitrogen in their foliage, such as the legumes. There is some evidence that sulfur has a particular rôle in