Chap. XL] GENERAL SUMMARY. 223 



special pharmacopoeia would be necessary to describe the di- 

 versified effects of various substances on the leaves of 

 Drosera. 



In the tenth chapter it was shown that the sensitiveness 

 of the leaves appears to be wholly confined to the glands and 

 to the immediately underlying cells. It was further shown 

 that the motor impulse and other forces or innuences, pro- 

 ceeding from the glands when excited, pass through the 

 cellular tissue, and not along the fibro-vascular bundles. A 

 gland sends its motor impulse with great rapidity down the 

 pedicel of the same tentacle to the basal part which alone 

 bends. The impulse, then passing onwards, spreads on all 

 sides to the surrounding tentacles, first affecting those which 

 stand nearest and then those farther off. But by being thus 

 spread out, and from the cells of the disc not being so much 

 elongated as those of the tentacles, it loses force, and here 

 travels much more slowly than down the pedicels. Owing 

 also to the direction and form of the cells, it passes with 

 greater ease and celerity in a longitudinal than in a trans- 

 verse line across the disc. The impulse proceeding from the 

 glands of the extreme marginal tentacles does not seem to 

 have force enough to affect the adjoining tentacles; and this 

 may be in part due to their length. The impulse from the 

 glands of the next few inner rows spreads chiefly to the 

 tentacles on each side and towards the centre of the leaf ; but 

 that proceeding from the glands of the shorter tentacles on 

 the disc radiates almost equally on all sides. 



When a gland is strongly excited by the quantity or 

 quality of the substance placed on it, the motor impulse 

 travels farther than from one slightly excited; and if sev- 

 eral glands are simultaneously excited, the impulses from all 

 unite and spread still farther. As soon as a gland is excited, 

 it discharges an impulse which extends to a considerable dis- 

 tance; but afterwards, whilst the gland is secreting and ab- 

 sorbing, the impulse suffices only to keep the same tentacle 

 inflected ; though the inflection may last for many days. 



If the bending place of a tentacle receives an impulse 

 from its own gland, the movement is always towards the 

 centre of the leaf; and so it is with all the tentacles, when 

 their glands are excited by immersion in a proper fluid. 

 The short ones in the middle part of the disc must be ex- 



