1 86 FRESH-WATER BIOLOGY 



seldom found without an incrustation, while Vallisneria is never 

 found with it, although the two plants frequently grow side by side 

 and essentially under the same conditions. The leaves of Vallisneria 

 are very flexible and almost always bending with the current, 

 hence, a deposit of soHd matter is prevented. It seems probable, 

 however, that the physiological processes going on in the plant 

 determine largely whether or not an incrustation is to be formed. 

 The coatings are not firmly fastened to the leaf and may be easily 

 scaled off or loosened by bending the leaf. The presence of the 

 coatings seems to make little difference to the plant as the tissue 

 beneath appears of a healthy green color though frequently of 

 more delicate tint than the unincrusted areas of the leaf. 



In all cases known the substance of the incrustation has been 

 found chemically to be the neutral carbonate of lime, which, of 

 course, is insoluble. Microscopic examination by polarized light 

 has revealed the presence of minute crystals in the incrustation 

 formed on Chara and the same may possibly prove to be the case 

 with plants of other families. The chemistry of the formation of 

 this incrustation is 'not known. There is usually present in the 

 water the soluble bicarbonate of lime which by loss of carbon 

 dioxid is changed to the neutral or insoluble carbonate. Some 

 have supposed that as the plants withdraw carbon dioxid from the 

 water to use in the process of starch manufacture, this insoluble 

 neutral carbonate is formed and deposited on the leaf. This proc- 

 ess may be expressed chemically thus: 



Soluble Insoluble 



CaH2 (003)2 = CaCOa + CO2 + H2O 



Another explanation may be that the oxygen liberated by the plant 

 in starch making acts catalytically upon the bicarbonate to change 

 it to the neutral carbonate. The former process would more 

 likely occur in water containing a larger amount of the carbonate 

 in solution which would be precipitated except for the solvent 

 action of the carbon dioxid in the water. The latter process 

 would be more probable in water containing very small amounts 

 of the bicarbonate which would remain in solution in the absence 

 of the carbon dioxid. 



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