■i 



116 



TU 



fall to a low level and temperature is high, it is very difficult to secure 



trictly 



localised pinnular movements 



petioles ; 



extent in the case of pinnu 



or 

 action almost invariably being diffused along the 



petiole being followed by pinnular action in one or more 



even strictly localised movements in the primary 



of the rachis to some 



uon of the primary 



of the secondary rachises, 



stimulation," and sudden dep 



and in some cases by varying 



deg 



of 



vergence 



of movement subsequent to sudden dep 



of the latter themselves 

 of the 



Th 



5 primary petiole is no 

 to which the pinnules are exposed 



d i ffusion 



doubt mainly owing to the sudden upward pressure 



from the air during the course of the rapid descent of the leaf, and to the fact 



that, in the position of full depression, leverage comes in to make for convergence of 



the se 

 effects 



condary rachises. These agencies 

 under the conditions of relativ 



pable of giving rise to any appreciabl 



sly stable equilibrium present m connection with 

 excessive telluric and atmospheric humidity, but are liable to produce some effect under 

 conditions in which the drain upon the actively turgescent tissues of the motor organs 



is higher 



In any case 



j 



stimulation" is a factor affecting processes of local 



filtration, but it will naturally give rise to different ultimate effects according to the 

 degree to which the turgescent tissues are already exposed to active drain of liquid. 

 The upward pressure of the air on the rapidly descending laminar surfaces in cases 



where the primary movement is one of petiolar dep 



places the upward pressure 



which any primarily displaced pinnule normally exerts on its distally situated neighbo 



Just as primary petiolar dep 



is followed by an entire absence of pi 



movements under conditions of high humidity 



so 



in many cases under similar 



circumstances the elevating action of the primarily displaced pinnule is insufficient to 

 give rise to anything save a certain amount of flexion in the lamina on which it 



acts, without any evidence of any alteration 



pulvinar turg 



The mutual relat 



of the laminar surfaces of adjacent pinnules is amply sufficient 



to account for diffusion of movement as a mere result of diff 



of condit 



favouring filtration, so long as the movements follow a centrifugal direction 



The successive la 



in their mutual relations precisely correspond to the successive 



cards or blocks of wood in the old familiar childish game of "Jack going to market." 

 Primary displacement of one at one end of the series is all that is necessary to 



secure the propagation of movement along the entire line 



If the anter 



card 



or 



blocks be firmly supported,— if they be in a condition of stable equilibrium— an 



impulse from their posteriorly 



ated 



give 

 any 



rise to displacement under cond 



ghbours, which would amply suffice to 



of unstable 





quilibrium, will fail to produce 



appreciable effect. The propagation of movement along the series— the "irrit- 

 ability" of the apparatus— rises with the instability of equilibrium just as it does in 

 the case of the pinnules. Whilst this is so in regard to centrifugal diffusion of 



movements along the course of the secondary rachis 



be so simply accounted for; as there is no satisfactory evidence that 



centripetal propagat 



when 



cannot 

 in full 



diurnal positio 



each 



lamina exerts any considerable active downward pressure 





on the upper surface of the one behind it. It is possible, however, that such pressure 

 may, in certain cases at all events, be present in some degree, and in so far as it 



is, its sudden removal on the elevation of the depressant pinnule must, of course, be 

 equivalent to a rise in the relative strength of the ii 



the depressed pinnule, and 



m 



th 



ferior pulvinar parenchyma of 

 quently a factor calculated to give rise to disturbance 



quilibrium of position in the latter. The fact, however, that movement of 







