=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 rôles 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.