490 CHARLES A. DAVIS 



It is probable that the plants actually do precipitate calcium 

 carbonate, both by abstracting carbon dioxide from the water and 

 by freeing oxygen, which in turn acts, while in the nascent state, 

 upon the calcium salt and precipitates it, but -in water- containing 

 relatively small amounts of calcium bicarbonate the latter would 

 seem to be the probable method. 



The calcium salt is deposited in minute crystals, and by the 

 aggregation of these crystals the incrustation is formed on the 

 plants. The crystals are distinguishable as such only for a short 

 time on the newer growths of plants, but the incrustations are 

 said to show a recognizable and characteristic crystalline structure 

 when examined in thin section under a compound microscope 

 with polarized light. 



Not all aquatic plants in the same lake seem equally active 

 in the precipitation of mineral matter. Not even all species of 

 the same genera, even when growing side by side, will be coated 

 equally, a fact which seems to indicate some selective metabolic 

 processes not understood. Considering the precipitation of 

 calcium carbonate by plants as established, even if the exact 

 physiological and chemical processes by which this precipitation 

 is brought about, are not yet worked out fully, it is still neces- 

 sary to consider the constancy of the action and the sufficiency 

 of the agency to produce the extensive deposits of marl which 

 are known. 



If one confines his studies simply to the seed-producing 

 plants and other large vegetable forms which are conspicuous in 

 lakes during the summer season, while he will find them cov- 

 ered with a thin coating of manifestly calcareous matter, he will 

 at once be convinced that such work as these plants are doing is 

 but a small factor in the total sedimentation of the lake. On the 

 other hand, if a visit be made to a lake in early spring or late 

 fall, all plants of the higher types will not be found, so that it 

 becomes apparent that this agency is merely a seasonable one 

 and works intermittently. Farther study of the plants of the 

 same body of water, however, shows that the algse, the less con- 

 spicuous and entirely submerged plant organism, must be taken 



