=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.