aken up this subject in the potato, but it is well to repeat the
distinction between stems and roots. A thick, short rootstock provided
with buds, like the potato, is called a _tuber_. Compare again the corm of
Crocus and the bulb of Onion to find the stem in each. In the former, it
makes the bulk of the whole; in the latter, it is a mere plate holding the
fleshy bases of the leaves.

[Footnote 1: Gray's First Lessons, revised edition, 1887, page 42.]

2. _Movements of Stems.--_Let a glass thread, no larger than a coarse
hair, be affixed by means of some quickly drying varnish to the tip of the
laterally inclined stem of one of the young Morning-Glory plants in the
schoolroom. Stand a piece of cardboard beside the pot, at right angles to
the stem, so that the end of the glass will be near the surface of the
card. Make a dot upon the card opposite the tip of the filament, taking
care not to disturb the position of either. In a few minutes observe that
the filament is no longer opposite the dot. Mark its position anew, and
continue thus until a circle is completed on the cardboard. This is a
rough way of conducting the experiment. Darwin's method will be found in
the footnote.[1]

[Footnote 1: "Plants growing in pots were protected wholly from the light,
or had light admitted from above or on one side as the case might require,
and were covered above by a large horizontal sheet of glass, and with
another vertical sheet on one side. A glass filament, not thicker than a
horsehair, and from a quarter to three-quarters of an inch in length,
was affixed to the part to be observed by means of shellac dissolved in
alcohol. The solution was allowed to evaporate until it became so thick
that it set hard in two or three seconds, and it never injured the
tissues, even the tips of tender radicles, to which it was applied. To the
end of the glass filament an excessively minute bead of black sealing-wax
was cemented, below or behind which a bit of card with a black dot was
fixed to a stick driven into the ground.... The bead and the dot on the
card were viewed through the horizontal or vertical glass-plate (according
to the position of the object) and when one exactly covered the other, a
dot was made on the glass plate with a sharply pointed stick dipped in
thick India ink. Other dots were made at short intervals of time and these
were afterwards joined by straight lines. The figures thus traced were
therefore angular, but if dots ha