10 CHEMISTRY OF PLANT LIFE 



or potassium, in the so-called " alkali soils " (i.e., those which con- 

 tain 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 trans- 

 port of phosphorus, particularly from older to more rapidly grow- 

 ing parts of the plant. More recent investigations have shown, 

 however, that magnesium has other roles which are probably 

 more specific and more important that 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 carbo- 

 hydrate-formation. This view is supported by the observations 

 that when algae are grown in magnesium-free solutions they con- 

 tain 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 com- 

 pounds being known to be among the most destructive of the 

 " alkali soil " salts. Calcium salts are remarkably efficient in 



