lo^i ANATOMY AND PHYSIOLOGY OF 



tlie cellular tissue has been cut away from one side of tlie articu- 

 lation, Dutrochet Q'^Nouv. rech, sut Vendosmose/' 4^7), drew the 

 conclu&ion, that the movement of the leaf always occurred through 

 the cellular tissue of that side of the articulation which becomes 

 convex in the curvature^ expanding actively, while the tissue of 

 the side which becomes concave remained perfectly passive. As 

 already observed, the fact upon which this conclusion rested, is 

 not perfectly correct. A leaf which has had the upper side of its 

 articulation cut away, of course immediately rises up nearly per- 

 pendicular, but it does not remain in this position ; in a few days 

 it recommences the performance, more weakly though it be, of the 

 sleep-movements (sinking and rising). It is, therefore, clear that 

 the expansion of the under side of the articulation, produced by 

 removal of its antagonist,^ as a general rule, raises the leaf higher 

 than in the natural condition, yet at the same time that this ex- 

 pansion is not constant, but undergoes a daily increase and de- 

 crease. The same circumstance (only in less degree) presents itself 

 after the under side of the joint is removed. We must conclude 

 from this that the expansion of one side of the articulation plays 

 the principal part in the elevation and depression of the leaf, but 

 that in this motion the opposite side likewise undergoes a change, 

 and, indeed, a relaxation. 



If the said view of Dutrochet was not wholly correct in regard 

 to the sleep-movements, still less was it in reference to the irritable 

 movements. It is clear that if the depression of a leaf resulting 

 from irritation is caused by the active expansion of the upper 

 side, overcoming the resistance of the under side, the tension must 

 inciease in the whole articulation, and the latter must become 

 rigid. Now, Brucke (" I c." 440) shewed that the articulation of 

 an irritated leaf becomes in no slight degree relaxed when this is 

 depressed ; we must, therefore, assume that the motion arising 

 from irritation depends not upon an increased expansion of the 

 upper side of the joint, but principally upon a relaxation of its 

 ^under side. This is also borne out by the circumstance, that a 

 leaf of which the upper side of the articulation has been cut away, 

 sinks (although not to the same extent as an uninjured leaf) when 

 irritated, which would be impossible if the movement had its 

 source solely on the upper side. 



This relaxation, as Brucke also shewed, either does not occur at 

 all in a sleeping leaf, or at least in a far weaker degree than in an 

 irritated leaf. Hence it is clear, as I formerly remarked on other 

 grounds (" Ueh. die Rei^harleit der Blatter von Robinia/' — Mohl, 

 " Verm. Schrifi/'), that the movement of irritability is not identi- 

 »cal with the sleep movement. Evidence of this is also offered by 

 the circumstance, that a sleeping Mimosa leaf is at least quite as 

 sensitive to irritation as a waking one, and perfoims the move- 

 ments of irritability very rapidly and to quite as great an extent. 



Concerning the internal occurrences, on which the relaxation 



