Rev. P. Keith on the Susceptibilities of Living Structures, 24-9 



quished; for they not only diverged widely from the supposed 

 lU at extreme temperatures, but they required the common 

 mercurial and the common air scale to differ about 5 at J 22 

 Fahrenheit; whereas at that point, the most emment ot the 

 French chemists declare they could find no difference at all; 

 and in their decision Mr. Dalton now acquiesces. 



XXXIV. Of the Susceptibilities of Living Structures. By the 



Rev. Patrick Keith, F.L.S. 



[Concluded from page 174.] 



WE have said that the sensible and voluntary susceptibi- 

 lilies of living fabrics are peculiar to animals. Yet they 

 have been claimed for plants also, and the claim has been ad- 

 vanced by botanists of the highest celebrity, ^^ well as by 

 physiological inquirers of no mean reputation. Dr. Watson, 

 Bishop ofLlandaff; Dr. Darwin, the ^^^^lior of The Loves of 

 Plants; Dr. Percival of Manchester ; and Sir J. L. Smith, the 

 founder and late President of the Linnsean Society of London, 

 —have each of them been advocates for the doctrine ot vege- 

 table sensation. Now a doctrine supported by writers ot such 

 eminence and distinction must surely have something in it, 

 if not very convincing, at least very plausible.— Let us see on 

 what foundation the doctrine is made to rest. 



One of the several arguments adduced in defence ot the 

 doctrine, is that of the fact of the excitability of plants by the 

 natural stiimdi of light and of heat, and, as it is also alleged, 

 of cold and of darkness.-lst. A plant placed in a dark room 

 with a small and single aperture for the admission ot light 

 will bend itself towards that aperture. The cutting ot a po- 

 tatoe left to vegetate in a cellar, or other dark apartnient 

 where there is but litde access of light, will be found to be a 

 very ffood subject of experiment— 2ndly. Leaves, it not pre- 

 vented by the operation of some accidental or intentional 

 cause, do uniformly turn their upper surface to the light. 

 Look at the leaves of trees trained to a wall, and you will hnd 

 the upper surface uniformly directed to the light-on a south 

 wall to the south-on a north wall to the north; and it tlie 

 leaf is forcibly bent so as to turn that surface towards the wall, 

 and is compelled to remain so for any given length ot time, it 

 will soon resume its primitive position upon regaining its li- 

 berty.— 3rdly. The leaflets of winged leaves, which expand 

 during the light ot day, begin again to droop and to bend down 

 about sun-set, or during the deposition of the evening ( ew, till 

 they approach or meet together on the under side ct the leui- 

 stalk; or tocloseand told themselves up till they approach or 

 N. S. Vol. 1 1. No. 61. April IS 32. '^ K meet 



