PLANT NUTRIENTS 9 



itself, have some beneficial or deleterious influence which gives to 

 the compound as a whole some important effect 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 hi some way con- 

 nected 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 algae 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 role 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, 



