de Vries, 'Arbeiten des Bot. Instituts in Wurzburg,' Heft iii. pp.
331, 336. See also Sachs ('Text-Book of Botany,' English
translation, 1875, p. 770), who concludes "that torsion is the result
of growth continuing in the outer layers after it has ceased or begun
to cease in the inner layers."
{7} Professor Asa Gray has remarked to me, in a letter, that in
Thuja occidentalis the twisting of the bark is very conspicuous. The
twist is generally to the right of the observer; but, in noticing
about a hundred trunks, four or five were observed to be twisted in
an opposite direction. The Spanish chestnut is often much twisted:
there is an interesting article on this subject in the 'Scottish
Farmer,' 1865, p. 833.
{8} It is well known that the stems of many plants occasionally
become spirally twisted in a monstrous manner; and after my paper was
read before the Linnean Society, Dr. Maxwell Masters remarked to me
in a letter that "some of these cases, if not all, are dependent upon
some obstacle or resistance to their upward growth." This conclusion
agrees with what I have said about the twisting of stems, which have
twined round rugged supports; but does not preclude the twisting
being of service to the plant by giving greater rigidity to the stem.
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