Each stinging-needle tapers
from a broad base to a slender summit, which, though rounded at the end,
is of such microscopic fineness that it readily penetrates, and breaks
off in, the skin. The whole hair consists of a very delicate outer case
of wood, closely applied to the inner surface of which is a layer of
semi-fluid matter, full of innumerable granules of extreme minuteness.
This semi-fluid lining is protoplasm, which thus constitutes a kind of
bag, full of a limpid liquid, and roughly corresponding in form with the
interior of the hair which it fills. When viewed with a sufficiently
high magnifying power, the protoplasmic layer of the nettle hair is seen
to be in a condition of unceasing activity. Local contractions of the
whole thickness of its substance pass slowly and gradually from point to
point, and give rise to the appearance of progressive waves, just as the
bending of successive stalks of corn by a breeze produces the apparent
billows of a corn-field.
But, in addition to these movements, and independently of them, the
granules are driven, in relatively rapid streams, through channels in
the protoplasm which seem to have a considerable amount of persistence.
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