Among the various economic products of the vegetable kingdom,
scarcely any hold a more important place than barks, whether for
medicinal, manufacturing, or other purposes. The structure and
formation of all barks are essentially very similar, being composed of
cellular and fibrous tissue. The cell contents of these tissues,
however, vary much in different plants; and, for this reason, we have
fibrous or soft, woody, hard, and even stony barks. To explain
everything which relates to the structure of bark would lead us into
long details which our space will not permit. Briefly stated, the bark
of trees (considering, now, those of our own climate) consists of
three layers. The outermost, called the "cortical," is formed of
cellular tissue, and differs widely in consistency in different
species; thus, in the cork oak, which furnishes man with one of his
most useful commercial products, the cortical layer acquires
extraordinary thickness. The middle layer, called the "cellular" or
"green bark," is a cellular mass of a very different nature. The cells
of which it is composed are polyhedral, thicker, and more loosely
joined, and filled with sap and chlorophyl. The inner layer (next the
wood), called the "liber," consists of fibers more or less long and
tenacious. It is from the liber that our most valuable commercial
fibers are obtained. In some plants the fibrous system prevails
throughout the inner bark; but what we wish to refer to more
particularly at present is a remarkable example of the harder and more
silicious barks, and which is to be found in the "Pottery Tree" of
Para. This tree, known to the Spaniards as _El Caouta_, to the French
as _Bois de Fer_, to the Brazilians as _Caraipe_, is the _Moquilea
utilis_ of botanists, and belongs to the natural order _Ternstroeiaceæ_.
It is very large, straight, and slender, reaching a height of 100 feet
before branching; its diameter is from 12 to 15 inches; and its wood
is exceedingly hard from containing much flinty matter. Although the
wood of the tree is exceedingly sound and durable, the great value of
the tree to the natives exists in the bark for a purpose which, to say
the least, is a novel one in the application of barks--that of the
manufacture of pottery. The Indians employed in the manufacture of
pottery from this material always keep a stock of it on hand in their
huts for the purpose of drying and seasoning it, as it then burns more
freely, and the ashes can be gathered with more ease than when fresh.
In the process of manufacturing the pottery the ashes of the bark are
powdered and mixed with the purest clay that can be obtained from the
beds of the rivers; this kind being preferred, as it takes up a larger
quantity of the ash, and thus produces a stronger kind of ware. Though
the proportions of ash and clay are varied at the will of the maker,
and according to the quality of the bark, a superior kind of pottery
is produced by a mixture of equal parts of fine clay and ashes. All
sorts of vessels of small or large size for household or other
purposes are made of this kind of ware, as are also vases or
ornamental articles, many of which are painted and glazed. These
articles are all very durable, and are able to stand almost any amount
of heat; they are consequently much used by the natives for boiling
eggs, heating milk, and indeed for culinary purposes generally. A
brief glance at the structure of the bark will show how it comes to be
so well adapted for this purpose. The bark seldom grows more than half
an inch thick, and is covered with a skin or epidermis; when fresh, it
cuts somewhat similar to a soft sandstone, but when dry, it is very
brittle and flint like, and often difficult to break. On examination
of a section under the microscope, all the cells of the different
layers are seen to be more or less silicated, the silex forming in the
cells when the bark is still very young. In the inner bark the flint
is deposited in a very regular manner, the particles being straight
and giving off branches at right angles; that of the porous cells of
the bark, however, is very much contorted, and ramifies in all
directions. In the best varieties of the tree, those growing in rich
and dry soil, the silex can be readily detected by the naked eye; but
to test the quality of the various kinds of bark, the natives burn it
and then try its strength between their fingers; if it breaks easily
it is considered of little value, but if it requires a mortar and
pestle to break, its quality is pronounced good. From an analysis of
this singular bark, that of old trees has been found to give 30.8 per
cent of ash, and that of young 23.30 per cent. Of the different layers
of old bark, the outer gave 17.15 per cent, the middle 37.7, and the
inner 31. The wood of the tree, in comparison with the bark, is
relatively poor in silex, the duramen of an old tree giving only 2.5
per cent of silex.
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