424 Prof. Burdon- Sanderson and Mr. F. J. M. Page [Dec. 14, 



on the lobes and midrib respectively ; el for the external surface (fig. 6) and 

 il for the internal surface of each lobe ; em the external surface, and im 



the internal surface of the midrib, and p for the petiole. We shall 

 further use the capital letter P for potential, and the capital letter Y for 

 variation of potential, emV for the potentials of the outer surface of the 

 midrib, and ^mP— _pP for the difference of potential between that surface 

 and the surface of the petiole, in other words, for the result of an ob- 

 servation made with the fixed contact at the petiole and the movable 

 contact at em. 



a. Mechanical Excitation. — In four leaves {a, b, c, d) observed in succes- 

 sion, the potentials and variations of the external surfaces of the midrib 

 and lobe were severally, in hundredths of the standard cell, as follows : — 



€mP-pP 



€ZP-i?P 1-6 1-6 



emV -5-0 -6-5 -4-2 -4-5 



eTV —2-0 —6-5 -4-0 —4*0 



The observations given, although taken in succession, exhibit great 

 differences of result. They serve to exemplify the general statement 

 that, as regards the external surface, the variation is greatest near the 

 midrib. 



AVhen the variation is compared on opposite and corresponding points, 

 it is almost always found that the external variation exceeds the internal. 

 Thus in nine observations the elY and ilV were respectively --6*5 (elY) 

 and —2-0 (UV), -4-5 and 0, —6-5 and —1-2, — 3-5 and —2-0, -3-5 and 

 _l-8, —6-5 and —3-0, — 4-5 and —2-2, — 5'5 and — 2-o, —4-5 and —2-5. 

 The following were the actual results obtained in two of these cases in 

 which successions of alternate measurements were made : — 



Leaf a (12 measurements) ?7P-^;P= - 1'2, ^ZP-^P = 0. 

 elY=-S'6, -4-0, -4-2, -4-0, -4-0, -4-5. 

 ^7y=-l•5, -1-7, -1-6, -1-8, -2-2, -2-2, 

 Leaf h (20 measurements) elV -^^P^ -1-0 - 1-5, t7P-^^P= -3-5. 

 elY= -5-0, -6-2, -6-5, -o'o, — 5-1, -5-0, -4-8, -49, -4-9, -4*9. 

 i7y=-2-0, -2-0, -2-0, -3-0, -3-0, -2-1, -2-5, -2'o, -2-5, -2-4. 



