ic 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 algae 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 roles 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 proportio