382 VEGETABLE IRRITABILITY. 



exactly the same as if the severance were complete ; no further effect 

 is obtained on separating the parts. 



" 3. Effect of constant current directed through the petiole on the 

 leaf-current. — If the leaf is placed on the galvanometer electrodes as 

 before, and the petiole introduced into the circuit of a small Daniell, 

 a commutator being interposed, it is found that on directing the 

 battery-current down the petiole {i.e. from the leaf), the normal deflec- 

 tion is increased ; on directing the current towards the leaf, the deflec- 

 tion is diminished. 



" 4. Negative variation. — a. If, tiie leaf being so placed on the 

 electrodes that the normal leaf-current is indicated by a deflection 

 leftwards, a fly is allowed to creep into it, it is observed that the 

 moment the fly reaches the interior (so as to touch the sensitive hairs 

 on the upper surface of the lamina), the needle swings to the right, 

 the leaf at the same time closing on the fly. 



" 6. The fly having been caught does not remain quiet in the leaf; 

 each time it moves the needle again swings to the right, always 

 coming to rest in a position somewhat farther to the left than before, 

 and then slowly resuming its previous position. 



" c. The same series of phenomena present themselves if the sensitive 

 hairs of a still expanded leaf are touched with a camel-hair pencil. 



" d. If the closed leaf is gently pinched with a pair of forceps with 

 cork points, the effect is the same. 



" e. If the leaf-stalk is placed on the electrodes as before, with the 

 leaf attached to it, the deflection of the needle due to the stalk-cur- 

 rent is increased whenever the leaf is irritated in any of the ways 

 above described. 



"/. If half the lamina is cut off, and the remainder placed on the 

 electrodes, and that part of the concave surface at which the sensitive 

 hairs are situated is touched with a camel-hair pencil, the needle 

 swings to the right as before." * 



Species of the genus Drosera (Sundew) exhibit excitability in the 

 leaves, with a certain amount of contractility. The leaves are of 

 various forms, some narrow and elongated, others spathulate or 

 rounded, aR bearing on their surface and at their edges beautiful 

 glandular hairs, with a spiral cofl. in their interior, and a globular apex 

 containing peculiar secretions (fig. 661). They are also insectivorous. 

 "When insects alight on the leaves they are entangled by the viscid 

 matter of the glandular hairs, which gradually close upon them, and 

 prevent their escape. The apex of the leaf turns inwards, so as more 

 effectually to secure the prey. This is well seen in some Australian and 

 African species. Droseras are by no means so excitable as the Dioneea, 

 but they seem to act in a similar manner upon insects. Small por- 

 tions of flesh placed upon the leaves induce movements of the hairs ; 



» Proceedings of the Royal Society of Loudon, Nov. 20, 1873. 



