MOTOR 



I 



which afford special facilities for filtrative escape of fluid. Under continued exposure, 

 however, an assimilatory struggle for the remaining supply of water is once more established, 

 and the tissues making for the diurnal position once "more undergo a relative inciv:^.. 

 in strength. This for some time enables them to give rise to a partial resumption of the 

 diurnal position even under the new conditions of transpiratory lot*; but, as the latter 

 continues, they again become weakened, and the various parts of the leaf gradually and 

 finally assume the positions corresponding to the passive structural str< ngth of the various 

 tissues entering into their composition. 



In the afternoon of the same day, supposing that it has been one of 



ihine, and 



pecial interfering factors have come into play, the conditions boil 



of the tissues themselves and of their environment are very different from what tlu v 



! 





were in the early morning. The tissues have for so long been exposed 



piratory loss as to ensure a great diminution, if not a total expenditure, of any surplus 

 of water-content beyond the amount necessary to meet the immediate demands of the 

 active elements, and atmospheric conditions are such as to imply much more active 

 transpiratory loss than that which was present in the morning. Under the circumstances, 

 it is only natural that any phenomena of movement dependent on alterations in the 

 relations of supply and loss of water should show certain modifications in character, and 

 that these modifications should be of the nature of those which are actually present 

 The supply of reserve-water within the water- conducting system being greatly decreased 

 or practically exhausted, and transpiratory loss being greatly increased, the niaintenaiico 

 of the diurnal position is secured only by means of immediate expenditure of the water 

 furnished by the stream traversing the water-conducting system from the roots; and 

 hence separation from the axis implies an immediate and very considerable drain upon 

 the active tissues. This secures proportionately rapid and complete action in the 

 pinnules and considerable movement in the secondary rachises. Under such circumstances 

 the recovery of the diurnal position is naturally less complete, exposure to direct sun- 

 shine is followed by more rapid movements and more frequently by abrupt, spreading 

 movements, and the secondary assumption of the nocturnal position is not only much 

 more complete than it was in the morning, but is also final and is never succeeded by 

 any symptoms of secondary recovery. 



• In dealing with these phenomena of movements in leaves and shoots when separated 

 from the axis and deprived of root-supply of water, it has been pointed out that exposure 

 to direct sunshine does not normally give rise to abrupt, but only to insensibly progressive 

 movements. The exposure necessarily implies just as much increased transpiratory los« 

 as it does in the case of detached leaves, and yet the occurrence of sudden movements is 

 quite exceptional in place of being a frequent or, under certain circumstances, a constant 

 event, as it is in the latter. This is readily explicable on account of the fact that leaves 

 whilst they are attached to the axis are not solely dependent on inherent water-supply, 

 but are permanently practically in a condition parallel to detached leaves containing a 

 large amount of reserve- water. The continuous supply of water ascending from the 

 roots replaces the local reserve of water in detached leaves in which only slow insen- 

 sibly progressive movements occur. In neither case is there any normal tendency to 

 rapid, sudden movements, because the sudden increase in transpiratory loss is to a great 

 extent equilibrated by the presence of an ample supply of water. The sudden increase 

 renders the maintenance of the maximal diurnal position impossible, but the loss in 



