91 
191 2-13. j Studies on Periodicity in Plant Growth. 
rate is attained, then follows a fall in the rate to a minimum immediately, 
or occupying again about four days, and so on repeatedly. 
In an organ of limited growth, the crest of successive four-day waves 
is progressively higher, culminating in a maximum which represents the 
acme of the grand period ; then a series of waves of successively less height 
follows until the organ ceases to grow. 
Frequently after the crest of the grand period is reached there follows 
a sudden diminution in the rate, succeeded by a long undulation, composed 
of a gradual acceleration to the fourth day and a gradual fall to the eighth 
(Plate I., fig. 1). 
The maximum of the long wave may occur either during a night or a 
day period, and thus in the premaximal phase of the grand period the 
maxima of the successive long waves may occur during the day, while their 
position is reversed in the post-maximal period. The converse is also found. 
An interesting feature brought out by the research up to the present 
is the widespread occurrence of the four-day periodicity — it being found 
alike in leaf of Cryptogam and the various organs of Phanerogams. 
Further, that it occurs in roots in light and darkness indifferently, is 
evidence that light is not a necessary condition here any more than it 
appears to be for the diurnal periodicity of roots. 
This present research only aims at recording the existence of this 
special periodicity and giving details regarding it. The extent to which it 
is influenced by external factors has yet to be accurately determined. On 
this point some knowledge may be gained from the results of experiments 
now in progress. 
Experimental Evidence of Four-day Periodicity. 
1. Tulip Leaf (Plate I., fig. 1). — This record was taken by means of the 
simple auxanometer from 25th March to 23rd April — a period of twenty- 
nine days. The natural condition during that period approximated to 
fourteen hours light and ten hours darkness per twenty-four hours period. 
An analysis of the curve shows the existence of five waves culminating on 
29th March, 2nd-3rd April, 8th-9th May, llth-12th May, and I7th-18th 
May respectively ; in all but the first the wave reached its highest during 
the night period. The waves occupied four, five, six, five, four, four days 
respectively, the third including the climax of the grand period of this 
organ of limited growth. 
During the period of slow fall in the growth rate following the 
maximum of the grand period, that is, during the post-maximal phase, there 
was frequently observed the occurrence of one or a series of waves extend- 
