ELEMENTARY CONSTITUENTS OF THE FOOD OF PLANTS. 
(or perhaps always) of calcium sulphate. This salt is probably, as Holzner first 
pointed out \ decomposed by the oxahc acid which is formed in the plant itself, and 
the insoluble calcium oxalate is thus formed, while the sulphuric acid parts with its 
sulphur to form the organic compounds which have been mentioned. 
Iron'^ (often accompanied by very variable quantities of Manganese) is indis- 
pensable for the production of the green colouring substance of chlorophyll, as is 
shown by experiments on vegetation ; and since the green organs which contain 
chlorophyll form organic substances out of water and carbon dioxide, the 
importance of this element for the hfe of the plant is very evident, although 
extraordinarily small quantities of it are sufficient for this purpose. It may be taken 
up by the plant in the form of the chloride or sulphate or of some other 
compound. If larger quantities of solutions of iron become distributed through 
the tissues, the cells quickly die. Although small quantities of iron are essential 
for producing the green colour of chlorophyll, it is nevertheless uncertain whether 
the green colouring substance itself contains iron as an integral constituent of its 
chemical formula^. 
Potassium is as essential for the assimilating activity of chlorophyll as iron for 
its production. Nobbe* has recently shown that if food-materials otherwise com- 
plete but possessing no potassium are supplied to plants (as Buckv>^heat), they 
behave as if they were absorbing only pure water instead of the solution of food- 
material. They do not assimilate and show no increase in weight, because no 
starch can be formed in the grains of chlorophyll without the assistance of 
potassium. The chloride is the most efficacious form in which potassium can 
be offered to Buckwheat ; the nitrate comes next to it. If the potassium is offered 
only in the form of sulphate or phosphate, a very evident sickliness is apparent 
sooner or later, which results from the starch w^hich is formed in the grains of chloro- 
phyll not passing into the growing organs and thus becoming available for purposes 
of vegetation. Sodium and Lithium cannot replace potassium physiologically, be- 
cause the former is simply useless to the plant, while the presence of the latter in 
the cell-sap is injurious to the tissues. 
Phosphorus^ Chlorine, Sodium, Calcium, and Magnesium have, as far as is yet 
known, no definite relation to special physiological purposes. The constant occur- 
rence however of compounds of phosphoric acid in company with albuminoids, as 
well as of potassium salts in organs containing starch and sugar, points towards 
definite relations which they may possess to those chemical processes that imme- 
diately precede the processes of construction in plants. A large part of the calcium 
taken up by plants is, as has been mentioned, precipitated by oxalic acid, and 
remains inactive. The importance of calcium must therefore be sought partly in 
its serving as a vehicle for sulphuric and phosphoric acid in the absorption of 
^ Holzner, Ueber die Bedeutung des Oxalsäuren Kalkes, Flora, 1867. — Hilgers, Jahrb. für wiss. 
Bot. vol. VI. p. I. 
^ For special proof of the importance of iron see my Handbuch der Experimental-Physiologic, 
p. 142. 
^ [On the chemical composition of chlorophyll, see infra.'] 
* Nobbe, Schröder, and Erdmann, Ueber die Organische Leistung des Kaliums in der Pflanze ; 
Chemnitz, 1871. 
