1882.] on the Excitability of Plants. 151 



mecliauical effect which follows it, and of the state of suspended 

 excitability) is governed by the temperature at which the observation 

 is made. The first of these propositions may be illustrated by 

 arranging an experiment in such a manner that a ventricle at 10^ C. 

 receives two excitations (induction shocks) at an interval of about 

 a second. Both are effectual, but, if the interval is in the slightest 

 degree shortened, the second fails, for it falls within the period of 

 suspended excitability. The proof of the second proposition can of 

 course only be obtained by series of measurements of the time 

 occupied respectively by the electrical disturbance and by the con- 

 traction, at different temperatures ; but when successive observations 

 are taken of the same ventricle at temperatures which differ by several 

 degrees, the contrast is very readily appreciated.* 



The experiments you have seen this evening may, I trust, have 

 served to illustrate the main facts of animal excitability sufficiently 

 to enable us to proceed to the subject which more specially interests 

 us — that of the excitatory phenomena of plants. 



Part II. 



The number of plants which exhibit what is often called irritability 

 is very considerable. I will not weary you with even enumerating 

 them. You will see from the table that they are distributed among a 

 number of natural orders, so that one might be inclined to suj^pose 

 that in this respect no relation could be traced between the physio- 

 logical endowments and the morphological characters of a plant. 

 That it is not so we have abundant evidence. Thus, in the same genus 

 we may find all the species excitable, though not in the same degree. 

 The extreme sensitiveness of the Chinese Oxalis, formerlv called 

 Bioplnjtum sensitivum, because it was supposed to be particularly alive, 

 appears in a less degree, but equally distinctly in our own wood-sorrel, 

 as well as in the Tree Oxalis of Bengal — the Carambola,t which is 

 described in an interesting letter addi-essed by Dr. Eobert Bruce to 

 Sir Jos. Banks, and j)ublished in the 'Philosophical Transactions.' 

 Again, in the same order, as, for example, among composite plants, 

 we may have the Thistles, Knapweeds, and Hawkweeds, all showing 

 excito-contractility in the same way, although the plants do not at 

 all resemble each other in external appearance. In order to make you 



* To illustrate the influence of temperature two ventricles were projected on 

 the screen, of which one was in contact with a lacquered metal surface at 10° C. 

 the other at 15° C. In the latter case the time occupied in the contraction was 

 about half a second shorter than in the former. As regards the period of 

 suspended excitability, it was fir^t shown that at 10° C. the second of two 

 excitations was ineffectual, but by raising the temperature two or three degrees, 

 the state of things was so changed that both excitations were followed by a con- 

 traction, the refractory period, like that of systole, being shortened by the 



warming. 



t " An account of the Sensitive Quality of the tree Averrhoa Carambola." By 

 Kobert Bruce, M.D. (' Phil. Trans.,' vol. Ixxv. p. 356.) 



