NA TURA L HIS TOR Y OF MA RL 489 



in flakes, while the tissues beneath show no injury from being 

 deprived of it, and again, as has already been pointed out, the 

 same^species of plants in some sections of the country do not 

 have any mineral matter upon them. The deposit is formed 

 incidentally by chemical precipitation upon the surface of the 

 plants, probably only upon the green parts, and in performance 

 of normal and usual processes of the plant organism. 



All green plants, whether aquatic or terrestrial, take in the 

 gas, carbon dioxide, through their leaves and stems, and build 

 the carbon atoms and part of the oxygen atoms of which the gas 

 is composed into the new compounds of their own tissues, in the 

 process releasing the remainder of the oxygen atoms. Admit- 

 ting these facts, which are easily demonstrated by any student 

 of plant physiology, we have two possible causes for the forma- 

 tion of the incrustations upon plants. 



If the calcium and other salts are in excess in the water, and 

 are held in solution by carbon dioxide, then the more or less 

 complete abstraction of the gas from the water in direct contact 

 with plants, causes precipitation of the salts upon the parts 

 abstracting the gas, namely, stems and leaves. But in water 

 containing amounts of the salts, especially of the calcium bicar- 

 bonate, so small that they would not be precipitated if there 

 were no carbon dioxide present in the water at all, the precipi- 

 tation may be considered a purely chemical problem, a solution 

 of which may be looked for in the action upon the bicarbonates, 

 of the oxygen set free by the plants. Of these calcium bicar- 

 bonate is the most abundant, and the reaction upon it may be 

 taken as typical and expressed by the following chemical equa- 

 tion : 



CaH 2 (CO,), + O = H 2 + CaC0 3 + C0 2 + O donate | + 



. calcium ) , carbon ) . 

 oxygen = water + rarhm , fp + , in • - + oxygen , 



carbonate \ ' dioxide 



in which the calcium bicarbonate is converted into the normal 

 carbonate by the oxygen liberated by the plants, and both carbon 

 dioxide and oxygen set free, the free oxygen possibly acting still 

 farther to precipitate calcium monocarbonate. 



