TRANSACTIONS OF SECTION D. 743 



they fail to expand completely after stimulation, until at length they 

 remain completely closed. 

 Effect of physostigmin : — 



a. On the main pulvinus. — The effect is gradually to diminish the extent of 



the recovery of the pulvinus after stimulation, until eventually the pul- 

 viuus retains the position characteristic of stimulation. 



b. On the leaflets. — The closing movement on stimulation becomes less and 



less marked, until finally they make no movement at all, but remain 

 open. 



Explanation of remits. — These results are readily intelligible when they are 

 considered in connection with the effect of the normal alternation of day and night. 

 From the researches of Briicke and of ISIillardet it is known that the tension in the 

 plant as a whole — that is, the state of expansion of its cells — diminishes during 

 the day and increases during the night. The closing of the leaflets when evening 

 comes on is the result of the commencing increase of tension or expansion. The 

 petiole also rises in the evening, as Pfeffer has shown, when the leaflets and 

 secondary petioles are removed ; bat when these are present the petiole is mechani- 

 cally depressed for a time, though even then it rises during the night. The leaflets 

 remain closed during the night. During the day the leaflets remain open, and the 

 petiole sinks to a more or less nearly horizontal position. 



The effect of atropin is just that of darkness ; it causes the leaflets to close and 

 the petiole to maintain a horizontal position, even when stimulated — that is to say, 

 it promotes the tension, or expansion, of the cells. 



Physostigmin, on the contrary, causes a diminution in the state or expansion — 

 or, in other words, a state of contraction — as is indicated by the position taken up 

 by the leaflets and by the petiole under its influence. Its effect is similar to that 

 of light. 



It must be borne in mind that, inasmuch as these observations were made 

 during the day-time, the effect of light must be taken into consideration. The 

 effect of light is antagonistic to that of atropin, but it coincides with that of phy- 

 sostigmin. Hence the effect of atropin is not so marked as it probably would be 

 in darkness. 



It was found possible to cause the atropin position to be replaced by the phy- 

 sostigmin position by transferring a branch from one solution to the other. 



The conclusion to be drawn is that it is the protoplasm which is the active 

 agent in the movement of the leaves, and not either the cell-wall or the cell-sap. 

 It is not conceivable that either the physical properties of the cell-wall or the 

 osmotic properties of the cell-sap should be affected in such opposite ways by these 

 alkaloids. 



Whilst making the above observations I noted some points which are of general 

 importance in the physiology of the movements of the leaves of Mimosa, and which 

 appear to have been overlooked in the descriptions given of the movements imder 

 various conditions : — 



1. The fall of the petiole is in no case caused by artificial darkness during the 

 day-time, but takes place only in the evening when the general tension diminishes. 



2. The secondary petioles are likewise unaffected by darkness during the day- 

 time, and converge only in the evening. Their movement is dependent on the 

 general diminution of tension. 



3. The secondary petioles are sensitive to mechanical stimulation only when the 

 leaf is young. 



6. On FlagclJa of Calamus. By Professor F. O. Bower. 



7. Note on the Stomala and Ligules of Selaginella. 

 Btj Professor McNab, M.I)., F.L.S. 



At the meeting of the Dublin Microscopical Club on March 20, 1884, the 

 author exhibited leaves of Selaginella densa, Hort. Sim., and S. Poulteri, Ilort. 



