IN THE MOTOR ORGANS OP LEAVES 



109 



degree, and the secondary rachises throughout exhibit little or no movement. In the 

 afternoon the pinnules assume the noetural position much more rapidly and completely 

 primary recovery is much more limited, rapid resumption of the nocturnal pontic on 

 exposure to direct sunshine occurs more frequently and much more completely; kh ndan 

 recovery does not follow continued exposure, and the secondary rachiaes exh 

 rapid and much more considerable movements of convergence. 



In order to explain these experimental results on the ordinal 0y accepted theory of tin' 



of the movements in Mniwsa pudica, it must be assumed eithef that separation 



bit more 



from the axis and exposure to the sun's rays are stronger stimuli 



pro topi 



contraction in the afternoon than in the morning or that tho protoplasts ore more 

 irritable in the afternoon than in tho morning; that separation {rem the ax.^ rend, 

 the contractile protop^sts more irritable or more contractile than they ai whilst 

 retaining their normal relation to it; that exposure to direct sunshine is, in the morn- 

 ing, only a temporary stimulant to contraction but in tho afternoon a permanent one* 

 and, finally, that the protoplasts of the tertiary pulvini aro much more irritable than 

 those of the secondary ones. But any such assumptions have absolutely no foundation 

 beyond the belief, that the movements must be dependent on active contraction of 

 the protoplasts of the motor organs, and some of them are directly controvert* d by 

 facts. Separation from the axis and exposure to the sun's rays produce less effect in 

 the afternoon than in the morning if the air and soil, in place of having* become dryer 

 in the interval, have become more humid; and leaves or shoots separated from tho u\i 

 when the soil and air are alike excessively humid aro much /cs$ "iiritable" than 

 leaves or shoots attached to the axis when soil and air are botli very dry. 



The other theory can, at all events, point to tho existence of differences in tho 

 state of the tissues and of the environment corresponding with the differences in th<- 

 phenomena of movement at different periods. So far as the tissues are concerned, it is 

 evident that in the morning they must normally contain a greater bulk of fluid than 

 they do in the afternoon. They have not had time to lose much by transpiratory 

 loss owing to the relatively high humidity actually prevailing at tho time, and they 

 have previously been exposed to a prolonged period of abolished transpiration. Under 

 these circumstances they are likely to contain a considerable excess of water within 

 the water-conducting system beyond the bulk which is absolutely nec< seary to maintain 



active turgescence under the existing conditions of immediate transpiratory logs. The 



amount of surplus will, of course, vary very considerably under different circumstances- 



but where the humidity of the soil is relatively high, it may well attain such ; 

 level as to suffice, in coincidence with slight transpiratory loss, to maintain turg< -cence 

 in the active tissues for some time after a complete arrest is put on any further root- 

 supply by separation from the axis, and even in spite of actual active discharge of 

 water attending the latter. Under such circumstances there is no immediate local 

 demand for root supply, as the local water-conducting system has become a reser- 

 voir for a greater bulk of water than is immediately required to make good transpira 



y expenditure. It 



rprising to find that, where previous and exis- 



tent conditions of soil and air imply excessive supply and minimised lo* of watei 

 separation from the axis is attended either by an entire absence of movement, or 

 veiy feeble and limited movement only. Where the water-content is such that the 

 active escape of liquid is sufficient to give ri-e to a considerable fall in the liquid 

 tension present throughout the water-conducting system directly, or primarily throughout 



