87 
1912-13.] Studies on Periodicity in Plant Growth. 
MacMillan and Golden {Amer. Nat., 1891, vol. xxv. p. 462) found a daily 
periodicity in the potato tuber, and ascribed it to a correlative effect 
between tuber and aerial shoots. 
The factors which have been by different investigators regarded as 
variously influencing periodicity are as follows : — 
1. Intensity of illumination. The periodic variation in the intensity of 
the refrangible rays reaching a maximum about midday and proportionally 
retarding or inhibiting growth in length. 
2. Variations in temperature. According to Sachs, in order to influence 
growth appreciably the temperature variations must be large; in which 
case the growth curve in shoots follows the temperature curve. Pedersen 
( Arb . Bot. Inst., Wurzburg, 1874, p. 563) states, on the other hand, that un- 
less the optimum is exceeded by many degrees, variations of temperature 
produce no marked effect on growth of roots. 
3. Variations in humidity. This factor, combined with the action of 
light on the stomata, affects the transpiration, for the existence of a 
daily periodicity in which some evidence is available. 
4. .The daily variation in bulk of the constituent cells of the plant organ. 
This phenomenon, described by Kraus {Bot. Zeit., 1867), is the expression of 
the variations of the internal tensions and constitutes a tension periodicity. 
Vines regards it as the cause of the daily periodicity of growth in length 
of growing organs. 
5. Reciprocal interaction between root and shoot. 
Special points characteristic of this research are : — 
(1) The growth records are continuous for relatively longer periods than 
is usual in this kind of experimental work ; of about eighty experiments, 
the results of which have been graphically plotted, the duration varied 
from ten to forty or more days in different instances. 
(2) Instead of an arbitrary division of the twenty-four hours period 
into twelve hours day and twelve hours night, the periods of light and 
darkness of natural occurrence at the date of experimentation were utilised. 
Sachs’ well-known curve representing the results of an experiment on the 
Dahlia performed in May is calculated on the basis of an alternation of 
twelve hours light and twelve hours darkness for twenty-four hours, hut 
in nature at that period of the year the plant would be subjected to at 
least sixteen hours light and only eight hours darkness. This point is of 
some importance, because the light and darkness periods vary throughout 
the year, and results of experiments performed when the day is at its 
shortest may be expected to differ from those carried on when it is at its 
maximum length. 
