iv] THE WATER SOLDIER 51 



water 1 . It has been shown experimentally 2 that, if the chalky 

 deposit be carefully removed from the surface of a plant which is 

 stationed at the bottom of the water, it immediately rises to the 

 top. In nature, the rising of the plant in spring is brought about 

 by the relative lightness of the young leaves, due to the absence 

 of a surface layer of calcium carbonate. As these young leaves 

 become more and more numerous in proportion to the old 

 leaves with their heavy deposit, the specific gravity of the plant 

 becomes less and less, until at last it is lighter than water and 

 floats up to the surface. 



The incrustation of the leaves of Stratiotes is by no means 

 unique; it has long been known that aquatic plants living in 

 * hard ' water are liable to become covered with a chalky coat. 

 The generally recognised explanation is that, since calcium car- 

 bonate is scarcely soluble except in water containing carbonic 

 acid, the abstraction of carbon dioxide, by the green organs of 

 aquatics, leaves the chalk as a deposit on their surfaces. This 

 theory is due to Pringsheim 3 , who demonstrated the truth of 

 his view by a series of very delicate experiments, in which he 

 actually observed microscopically the deposition of crystals of 

 calcium carbonate upon the surface of moss leaves, algal fila- 

 ments, etc., immersed in water containing carbon dioxide and 

 calcium carbonate in solution. 



Owing to the curious mode of life of Stratiotes^ its youngest 

 leaves are usually entirely submerged, but when mature, they 

 may be submerged for part of the year but raised above the 

 surface for another part. It was formerly supposed that the 

 distribution of the stomates on the leaves could be directly 

 traced to the action of the environment. For instance, it has 

 been stated 4 that, in a single leaf which was partly submerged 



1 In justice to Nolle, it ought to be mentioned that he anticipated the 

 discovery that the rising and sinking of the plant was due to differences 

 in specific gravity between the old and young leaves, but he made the 

 mistake of supposing that the greater weight of the old leaves was due to 

 waterlogging. 2 Montesantos, N. (1913). 



3 Pringsheim, N. (1888). 4 Costantin, J. (i885 3 ) and (1886). 



42 



