CHAP, in.] the Movements of Plants. 561 



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 De Candolle, declared against the existence of 

 any such relation, but a similar idea w^as 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 days is multiplied by 



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