1876.] 



On the Leaf of Dionsea muscipuL 



411 



elevation and depression of the centre o£ gravity of each atmospheric 

 column. Herein may lie the reason why lateral expansive and contrac- 

 tive currents are found to exist. 



A reference made by Dr. Andrews, in the course of his inaugural 

 Address to the British Association at the recent Grlasgow Meeting, to the 

 work of G-eneral Menabrea, appears to give weight to this conjecture. He 

 says — "His great work on the determination of the pressures and ten- 

 sions in an elastic system is of too abstruse a character to be discussed 

 in this address ; but the principle it contains may be briefly stated in the 

 following words : — "When any elastic system places itself in equilibrium 

 under the action of external forces, the work developed by the internal 

 forces is a minimum." 



December 14, 1876. 



Dr. J. DALTON HOOKER, C.B., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were rioad : — 



I. " On the Mechanical Effects and on the Efectrical Distur- 

 bance consequent on Excitation of the Leaf of Dioncea musci- 

 pulaJ' By J. Burdon-Sanderson, M.D._, F.R.S._,^ Professor of 

 Physiology in University College, and F. J. M. Page, B.Sc, 

 F.C.S. Received November 23, 1876. 



Part I. — Mechanical Eeeects. 



The mechanism by which the leaf of Dioncea closes after mechanical 

 excitation has been already studied by Mr. Darwin and many other 

 naturalists. It was, however, necessary, in order to connect the electrical 

 phenomena which form the principal subject of this paper with this 

 mechanism, to study the successive changes of form which the leaf under- 

 goes in the act of closing. The investigations we have made relating to 

 this subject have brought to our knowledge facts which have an im- 

 portant bearing on the general question of the nature of the excito- 

 contractile process in plants and animals. 



The smooth green outer surface of a leaf of Dioncea in full vigour is 

 concave, and the marginal hairs are thrown back so that they are 

 nearly in the same plane with the lobe from the edge of which they 

 spring. If one of the sensitive hairs of a leaf in this condition is care- 

 lessly touched the leaf usually closes. If, however, a hair is touched 



VOL. XXV. 2 G 



