owing
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, 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 role 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
organ