12.1 



' IN THE MOTOE OEGANS OF LEAVES. 



fluctuations in the liquid contents of the latter, but here their relation to the occurrence 

 of propagation of movements ceases. Their excessive turgidity and their structure 

 render them effective agents m securing the loss of large quantities of liquid on inci- 

 sion and this provides for a disturbance in the p«K,xi*mt equilibrium between xnpplv 

 and loss of liquid generally. Their excessive turgidity secures a large discharge of 



liquid, and their high osmotic capacity, on winch their normal turgeseeme 



render them a source of excessive drain upon the generally available stock of HquiT A 

 sudden discharge of a large quantity of liquid from them implies a suddenly increas 

 demand upon the general water-supply, and, owing to their excessive osmotic capacity 

 they are well adapted to worst other active tissues in the struggle which must en •> until 

 a condition of liquid equilibrium be once more established. As has been already pointed 



depends must 



the presence of any extensive system of excessively osmotic elements is in' i, 8( lf 

 element favouring instability of equilibrium on the relations between general loss I 



pply of liquid in less highly osmotic tissues. When any g 



d 



turgidity 



within it, apart from any alteration in its functional activities, and con equently of i 

 osmotic capacity, the latter co-operates with active transju'ratory loss in (riving rise to a 

 great draught upon the general stock of fluid available for the maintenance of turges< en< 

 whilst the coincident fall of pressure probably induces additional hit rathe escape from 



> 



escat 



other active tissues. The " Beizleitendc system" is an apparatus through means of A\]ncl 

 the conditions of equilibrium between general supply and loss of liquid may bo 

 readily disturbed and, therefore, is an apparatus through means of which mov. uent 

 readily be indirectly determined ; but is not peculiar or specific in this respect for, as 

 we have already seen, similar effects may be produced by means of ehan- * taking 

 place within the wood 







y 



©v~ M*0."IIC 





Haberlandt affirms that any fluctuations of pressure within the system of seive-tube 





whether of a positive or negative nature, are capable of acting as stimulants to the con- 

 tractile tissue of the motor organs. But the phenomena attending the local application of 

 heat to mature, hard axes show that fluctuations of pressure within it are not necessarily 

 efficient factors in determining movements, and also that it is only negative fluctuations 

 in general liquid pressure that are anywhere efficient. In such cases wo have positive 

 fluctuations in liquid pressure arising within the sieve-tubes in both directions alike and 

 movements occurring only centrifugally, that is in the direction of diminished liquid- 

 pressure, within the water-conducting system. Negative fluctuations in liquid-pressure in 

 these cases occur in all distally situated areas in consequence of the obstructive expansion 

 of the gases of the water-conducting system; in cases of incision, on the other hand 

 similar fluctuations arise both centripetally and centrifugally owing to the sudden escape 

 of considerable masses of liquid from the tissues. In the first instance, movements occur 

 only centrifugally, in the second both centripetally and centrifugally ; in other words, in both 

 instances alike, the distribution of movements coincides with the distribution of negative 

 fluctuations in liquid-pressure. Such fluctuations may arise in various ways; they may 

 arise in consequence of escape of liquid from the system of sieve-tubes, of obstructions 

 arising in the water-conducting system of the wood, of sudden contraction in the 

 gaseous contents of the latter, of increased transpiratory loss, or of sudden arrest of 

 root-supply connected with separation from the axis; and in all cases alike they are 

 liable to be followed by the occurrence of spreading movements. Negative fluctuations 

 in general liquid-pressure are the specific determinants of all spreading movements which 



