i9i3" I 9 I 4-] Plant Growth-Rhythms. 103 



Edin.,' 1912-13, vol. xxxiii. pp. 85-102) led to the following 

 conclusions : — 



1. Daily periodicity in shoots continues to be manifested 

 even when the conditions are changed, as for example when 

 the shoots are subjected to constant darkness. 



2. Eoots under the natural conditions of constant darkness 

 exhibit periodic growth, which is found to continue when the 

 conditions are changed and the roots are subjected to alternate 

 light and darkness. 



3. The general conclusion was that an autonomic rhythm 

 underlay the daily periodicity of growth of root and shoot, 



Four-Day Periodicity. — Several years ago, while examining 

 some particularly long-growth records which extended con- 

 tinuously over periods of one to two months, I discovered 

 a hitherto unrecorded growth periodicity of greater extent than 

 the daily one, being on the average of four days' duration. 

 This was referred to in the communication to the Scottish 

 Microscopical Society already mentioned, and the results of a 

 fuller investigation were communicated to the Eoyal Society 

 (loc. cit.). 



This periodicity is generally present in the organs of the 

 higher plants, and appears to be of autonomic origin. It 

 manifests itself as a gradual acceleration of growth extending 

 over four days, when a maximum rate is attained, whereafter 

 follows a fall in the rate to a minimum, immediately or 

 slowly, occupying again four days, and so on repeatedly. 



General Conclusions. — 1. We may visualise the growth 

 of an organ as made up of a series of waves, every fourth 

 wave larger than the three preceding, and the successive 

 fourth waves progressively larger, until the acme of the Grand 

 Period of growth is attained. 



2. The living plant has an innate tendency to periodic 

 expenditure of energy in growth which finds expression in the 

 annual, diurnal, and four-day rhythms. These are influenced 

 and modified by variations in the condition of environment. 



[Numerous lantern-slides were shown to illustrate this 

 lecture.] 



