30 m., they still remained
horizontal, excepting one which was now inclined 20o beneath the horizon.
The terminal part, 10 mm. in length, had increased greatly in length during
the 23 h. 30 m., viz., to an average of 26 mm. Four control radicles became
slightly geotropic after the 4 h. 10 m., and plainly so after the 5 h. 45
m. Their mean length after the 23 h. 30 m. had increased from 10 mm. to 31
mm. Therefore a slight cauterisation of the tip checks slightly the growth
of the whole radicle, and manifestly stops the bending of that part which
ought to bend most under the influence of geotropism, and which still
continues to increase greatly in length.]
Concluding Remarks.--Abundant evidence has now been given, showing that
with various plants the tip of the radicle is alone sensitive to
geotropism; and that when thus excited, it causes the adjoining parts to
bend. The exact length of the sensitive part seems to be somewhat variable,
depending in part on the age of the radicle; but the destruction of a
length of from less than 1 to 1.5 mm. (about 1/20th of an inch), in the
several species observed, generally sufficed to prevent any part of the
radicle from bending within 24 h.
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