i9i3" I 9 I 4-] Plant Growth- Rhythms. 99 



far from this weem. Why is its site not known ? Simply 

 because it has been covered over and buried out of sight 

 by the detritus of centuries. The discovery of the Roman 

 settlements at such places as Newstead and Inveresk was 

 due to accidental excavation, which disclosed what had been 

 long hidden away. 



Ill — PLANT GROWTH-RHYTHMS. 



By Mr E. A. EOBEETSON, F.E.S.E., F.L.S. 



{Communicated, Dec. 17, 1913.) 



The plant organism receives energy from without, and expends 

 it in many ways — in movements, for example, particularly that 

 known as growth. Movement may be continuous or inter- 

 mittent, depending on the nature of the supply of energy, 

 the inherent construction of the organism, and the external 

 conditions. In the consideration of the subject, several ques- 

 tions are suggested. Is the supply of energy to the plant 

 continuous or interrupted ? Is the plant constructed for con- 

 tinuous or intermittent movement ? Is the expenditure of 

 energy in growth movement intermittent or continuous ? If 

 intermittent, is the movement of growth characterised by any 

 regularity representing a definite rhythm or periodicity ? 



It is a fact of common observation that the natural supply 

 of energy in the form of light and heat available to the plant 

 varies, seasonally, daily, hourly ; hence even if the organism 

 were constructed for continuous movement, the movement 

 would be intermittent unless the organism possessed some 

 device for energy storage to ensure continuance. The chemi- 

 cal compounds built up by the aid of light energy represent 

 not only plastic material but also a store of energy available 

 when the immediate supply from without is diminished in 

 amount. 



The activities of the living plant are only exhibited under 

 certain suitable external conditions, — supply of oxygen, mois- 



