THE VEGETABLE SECRETION OF SILICA. 



55 



sugar, gum. In the early stages of development, the major product is starch; but, as 

 maturity approaches, this is gradually changed to sugar ; and to gum when the period 

 of decay ensues, or the starch at once passes into the state of gum. So in the malting 

 of barley : the object there is to convert the starch into sugar ; but if the process of 

 germination be carried a little too far, the sugar begins to disappear, and is supplanted 

 by gum. The prolonged cookery of any farinaceous substance has always this ten- 

 dency ; so that biscuits not unfrequently contain a portion of gum, difficult of digestion, 

 with the starch which is capable of ready conversion into the material of the blood. 



Silica. The last secretion to which we shall now refer, is one of peculiar interest 

 vis., silica, or flint. This is a mineral substance ; and, apart from vegetable structures, 

 is so indestructible that the strongest chemical acid is required for its solution, and yet 

 it has structures so delicate that a stem of wheat can dissolve it with facility. It is not 

 pretended that vegetables have the power of producing flint, but only that they are 

 enabled to dissolve it in their juices, when water and other fluids alone cannot dissolve 

 it. This power seems to reside at the extremities of the rootlets, for it is impossible 

 that flint could be taken into their delicate tissues until it has been dissolved. The 

 sources of silica or flint, are 



1. The sand which is so largely met with in almost all kinds of soil, and which has 

 the further valuable property of permitting the rain to percolate to the roots of the 

 plant. Its composition, is chiefly that of silica, as may be familiarly inferred from its 

 essential presence in the manufacture of glass. 



2. From the flint nodules which are found in the chalk formations, and which 

 themselves are the productions of long-buried sponges, mosses, and minute animalcules. 



3. From the skeletons of animalcules which 

 still remain in the soil. These skeletons are com- 

 posed of flint, as may be proved from their non- 

 solubility in boiling nitric acid (Fig. 97). So 

 numerous are they that Richmond, in Virginia, 

 United States, is built upon a stratum eighteen 

 feet deep, and upwards of thirty miles in length ; a 

 stratum representing an innumerable number of 

 animalcules, when it is borne in mind that each 

 animalcule is almost too small to be seen by the 

 naked eye. Similar deposits also exist in the old 

 world. 



These skeletons are also found in other posi- 

 tions. Thus guano, a substance consisting of the 

 excrements of birds, contains vast numbers, 

 chiefly of three genera, Actinocyclus, Gallionella 

 (Fig. 97), and Coscinodiscus. A powdery sub- 

 stance is known in Germany as Berg Mehl, or 

 mountain meal, which is chiefly composed of Fig:. 9 7 - SilHous pkeletons of the Diato- 

 them. This is the produce of the strata through ^UT^^^^t^S- 

 which the mountain torrents run, and is brought lodtscus ch/peus, both found in guano. 

 down by the waters. From its resemblance to 

 flour, it is used in certain localities as an article of diet. 



4. From the remains of plants in the form of manure or otherwise, which contain 

 silica ; as, for example, the wheat straw. 



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