196 BIOLOGICAL LECTURES. 



needed to start movement in the benumbed ones as well as in 

 these. On several occasions, a behavior has been noted, both in 

 Oxalis dendroides and the common sensitive plant, that is worth 

 recording, whether the suggested explanation of the behavior 

 is the correct one or not. A bit of ice laid directly on the 

 pulvini of two opposite leaflets failed to excite them to motion 

 and even a pair beneath would only partially contract, while 

 those still further down would move in normal fashion. I 

 satisfied myself that the position and weight of the ice particle 

 offered no obstacle, and learned also that if within 20-30 

 seconds the ice be removed, the leaflets after an added interval 

 of 7-15 seconds fall backwards as if recently stimulated. We 

 believe a probable explanation here to be that during the latent 

 period of excitation, the ice had so lowered the temperature of 

 the subjacent cells as to benumb the protoplasm, which only 

 regained its contractile properties with returning irritability 

 after removal of the ice. 



We may next inquire whether a peripheral or centripetal 

 stimulus is propagated to the leaf base or to the stem more 

 rapidly than a centrifugal stimulus initiated at the basal leaflets. 

 The only plant that has hitherto been experimented on is 

 the common sensitive plant, but Dutrochet and Bert expressed 

 different views. Oxalis is much more convenient for the deter- 

 mination of this question. Each leaf carries 15-24 pairs of 

 leaflets. A particle of ice placed on the pulvini of the 

 middle pair, i.e., the tenth if there are 19 pairs, will excite all 

 the pairs above it within 15-17 seconds, but 21-23 seconds will 

 elapse before the lowest pair in such a leaf as the tenth from 

 the apex-bud closes. With the ice in varying positions and 

 on leaves of different age I have invariably found that a cen- 

 trifugal is more rapid than a centripetal impulse, and I have 

 studied many examples. But the records are less satisfactory 

 as regards simultaneous basal and apical initiation of excitation, 

 though the balance of experimental proof is in favor of centri- 

 fugal stimulus. 



As with sensitive plants in general, carbonate of ammonia is 

 a powerful stimulant, and its rapidity is proportioned to the 

 strength of the solution. When a small drop of a 20 ft 



