502 Reed. — The Value of Certain Nutritive Elements 
simple compounds. The fact that the processes of metabolism in the plant 
cell are relatively slow, and permit of more exact observation than in the 
animal cell, makes the former a very favourable object of research. 
De Saussure, in his classical ‘ Recherches chimiques sur la vegetation,’ 
placed on record his observations and experiments, which proved that the 
ash of plants contains the same mineral nutrients which they drew from the 
soil. He showed that terrestrial plants draw their mineral nutrients from 
the soil in the form of aqueous solutions, and that the ash of seaweeds 
contains only the salts which are present in sea water. In certain respects 
De Saussure went far beyond his contemporaries. He succinctly states, for 
example, that plants do not absorb salts in the same proportion as they 
exist in the soil solution. He also recognized the ability of plants to 
utilize very dilute solutions of mineral nutrients. 
II. Historical. 
i. Previous work upon the role of potassium. 
De Saussure, in the treatise already mentioned, established the 
necessity of potassium salts for the growth of terrestrial plants. Birner 
and Lucanus (’66), experimenting with oat plants in water cultures, gave 
the first proof that the element potassium is absolutely indispensable for 
flowering plants, and cannot be replaced by rubidium, caesium, sodium, 
lithium, or ammonium. The perfection of the water-culture method and 
consequent refinement of experimental methods enabled these investigators 
to obtain data of fundamental value. Their work was extended and con- 
firmed by G. Wolf (’68), who attempted to establish Liebig’s law of the 
minimum for plants grown in water culture. The results of Hellriegel (’67) 
on barley also confirmed the observations of Birner and Lucanus. 
Molisch (’96) showed that the related elements, rubidium, caesium, lithium, 
and sodium, could not be substituted for potassium in the physiology of 
Protococcus infusionum. Nobbe (’70) seems to have been the first in- 
vestigator to point out the necessity of potassium salts for the formation of 
carbohydrates in plants. Gaunersdorfer (’87) not only showed that lithium 
cannot be substituted for potassium but that it has a toxic action upon 
some plants. 
Loew (’98 a) has shown that rubidium may replace potassium in the 
nutrition of certain of the lower plants. Also that rubidium exerts 
a stimulating action on plants when all necessary nutrients are present. 
Benecke (’07) has also stated that rubidium and caesium within certain 
limits may replace potassium in the nutrition of Bacillus fluorescens and of 
B. pyocyaneus. 
In the presence of potassium, sodium has been shown also to produce 
increased growth in plants. Hellriegel (’98) found that plants grown in 
