88 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The importance in this research of the first characteristic is, that it has 
in consequence been possible to bring to light a hitherto unrecognised 
growth variation of greater extent than the diurnal one, extending on the 
average to about four days. 
In connection with the second, it has been found that there is a tendency 
for the time of maximal diurnal growth to vary according to the change 
in the length of the period of natural illumination per twenty-four hours 
throughout the year. 
(3) The experiments have been carried on in a special experimental 
greenhouse, in which it is possible to regulate the temperature within one 
degree of variation ; to control the illumination and regulate the humidity, 
although not to the same degree of precision as in the case of the 
temperature. 
For statistical purposes the following table gives the data used in 
calculating the relative duration of the periods of light and darkness per 
twenty-four hours for different months of the year. The figures are 
approximate, since an hour or more requires to be added in certain months 
for the additional illumination before sunrise and after sunset : — 
Sunrise. 
Sunset. 
Sunrise. 
Sunset. 
January 
8 am. 
4 p.m. 
July . 
4 a.m. 
8 p.m. 
February 
7 „ 
5 „ 
August . 
5 „ 
7 „ 
March 
6 „ 
6 „ 
September 
5 „ 
6 „ 
April 
5 „ 
7 „ 
October . 
6 „ 
5 
May 
4 „ 
8 „ 
November 
7 „ 
4 „ 
June 
4 35 
8 „ 
December 
8 „ 
4 „ 
In estimating the relative day and night growth rate for an experiment 
during April, for example, the growth increments from 5 a.m. to 7 p.m. 
were summed and divided by the number of hours of daylight, that is, 
fourteen ; for night, the sum of the growth increments from 7 p.m. to 
5 a.m. divided by ten gave the night rate ; while at midsummer the day 
and night figures are sixteen and eight respectively. In nature the retarda- 
tory factor — light — increases in magnitude, while the acceleratory factor- 
darkness — diminishes from spring to midsummer ; while the converse is the 
case in the latter half of the year. 
The four-day variation was first met with in organs of limited growth, 
but experiment has shown that it is not confined to such, and that 
organs of all kinds show it — leaves, shoots, roots, leaf stalks and 
flowering axes. 
