The work is issued in parts, each representing twenty-five species, and
selling with text at $5, expressage prepaid; the mounted specimens alone
at 25 cts. per species or twenty-five in neat box for $4. He has also
a line of specimens prepared for the stereopticon and another for the
microscope. They are very useful and sell at 50 cts. per species or
twenty-five for $10.]
Let each of the scholars take one of the sections of Oak and write a
description of its markings. The age is easily determined; the pith rays,
or _medullary rays_, are also plain. These form what is called the silver
grain of the wood. The ducts, also, are clear in the Oak and Chestnut.
There is a difference in color between the outer and inner wood, the older
wood becomes darker and is called the _heart-wood_, the outer is the
_sap-wood_. In Birds-eye Maple, and some other woods, the abortive buds
are seen. They are buried in the wood, and make the disturbance which
produces the ornamental grain. In sections of Pine or Spruce, no ducts
can be found. The wood consists entirely of elongated, thickened cells or
fibres. In some of the trees the pith rays cannot be seen with the naked
eye.
Let the pupils compare the branches which they have described, with a
stalk of Asparagus, Rattan, or Lily. A cross section of one of these shows
dots among the soft tissue. These are ends of the fibro-vascular bundles,
which in these plants are scattered through the cellular tissue instead of
being brought together in a cylinder outside of the pith.
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