tabled long-continued measurements of the longitudinal growth
of parts of plants, and gave an idea of constant irregularity of
growth, without suggesting any explanation of the causes which
produced it ; so indistinct were the ideas of observers on these
subjects even after 1850, that the majority of them proposed to
themselves the question, what difference there is between
growth by day and by night ; it did not occur to them that day
and night are not simple forces of nature, but different and
very variable complications of external conditions of growth,
such as temperature, light and moisture, and that such a mode
of putting the question could not possibly lead to the discovery
of the relations established by law, so long as the several
factors were unknown which are included in the conceptions of
day and night. Harting's essay of 1842 is superior to those
above mentioned, inasmuch as he distinctly endeavoured to
obtain from his measurements some definite propositions that
might be applied to the theory of the subject, and especially to
give a mathematical expression to the dependence of growth
on temperature, but his success in this particular point was not
great. The idea, that there must be a simple arithmetical
relation to be discovered between growth and temperature,
had been suggested by Adanson in the previous century, and
it found many supporters in the period between 1840 and
1860: but it should be observed that the term growth was
used in a loose and popular sense to sum up all the phenomena
of vegetation in one expression. Adanson had supposed that
the time occupied in the unfolding of the bud was determined
by the sum of the degrees of the mean daily temperature,
reckoned from the beginning of the year; Senebier, and at
a later time I)e Candolle, declared against the existence of
any such relation, but a similar idea was not only very
generally entertained after 1840, but it even came to be treated
as a probable natural law. Boussingault had pointed out that
in the case of cultivated plants in Europe and America, if the
whole period of vegetation expressed in clays is multiplied by
Page:History of botany (Sachs; Garnsey).djvu/581
Chap. III.]
the Movements of Plants.
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