112 



i 



HE CAUSES OF FLUCTUATIONS IN TUEGESCENCE 



tions in the leverage of distal parts which may 

 or 



tissues. 



turgescence is a gradual one, and where root-supply is abundant and atmospheric humid- 

 ity is not excessively depressed, a modified diurnal position is shortly arrived at, the 

 precise nature of which varies with the relations between supply and loss of water, the 

 differences in osmotic properties of the opposed masses of pulvinar tissues, and any altera- 



have been induced. The reserve-water, 



the water continuously supplied from the roots, acts as a kind of buffer interposed 

 between transpiratory loss and the water of turgescence contained within the active 



The increased transpiratory loss must, of course, eventually tell on the amount 

 of water available for the up-keep of turgescence in all cases where root-supply is absent, 

 or is incapable of increasing in activity to the extent of entirely discounting the increased 

 loss; but in the presence of a large stock of reserve-water or of active root-supply it 

 only does so gradually, whereas in the cases of detached leaves, unprovided with much 

 reserve-water, an active drain sets in at once upon the turgescent tissues, and specially 



those which provide the greatest structural facilities for the redistribution of liquids. 



]Uit in certain cases rapid sudden movements do attend the sudden exposure of 

 attached leaves to direct sunshine. In hot dry weather, it not unfrequently happens that 

 when the direct sunshine first strikes upon a plant in the morning a certain number of 

 the youngest mobile leaves, in which full expansion of the pinnules has not yet been 

 established, presently show rapid movements of depression in their primary petioles. 



on 



The 



owing 



to the action of the same cause as that whicl 



movements in such cases may be 

 occasions sudden movements in detached leaves. They may be owing simply to the 

 sudden increase in transpiratory drain, acting upon the pulvinar tissues which make 

 for the diurnal position at a time when they are specially structurally weak owing to 

 their incomplete development; but it appears probable that, in part at all events, they 



owe 



their 



origin 



to the action of another factor. The delicate tissues of the terminal 



portions of the axis on which the youngest leaves are situated are, of course, very readily 

 affected throughout by sudden changes in temperature, and just as a sudden and con- 

 siderable fall in temperature must tend to establish a back-draught of liquid into the 

 water-conducting system, so a sudden and considerable elevation may lead to tem- 

 porary obstruction to its onward progress. Any sudden elevation of temperature must 

 occasion corresponding expansion in all the gaseous contents of the water- conducting 



system 



j 



and ?;if 



the expansion exceed certain limits, it must almost inevitably lead to 



more or less obstruction of the current of water ascending from the roots. It is not, 

 however, necessary to remain contented with theoretical considerations, for there is 

 conclusive experimental evidence which demonstrates not only the occurrence of such 

 obstruction, but also the fact that it is amply sufficient to give rise to the occurrence of 

 sudden and extensively propagated movements. If an open flame, or the points of a 

 heated pair of forceps, be applied to any point in the course of an axis of Mimosa 

 pudica, or, still better, if the sun's rays be accurately focussed upon it, very conspicuous 

 phenomena of movement presently manifest themselves in the leaves, unless under condi- 

 tions of excessive saturation of the soil and atmosphere. In the case of young, soft 

 shoots the treatment gives rise to conspicuous local change in colour and to more or less 



conspicuous discharge of liquid upon the surface, but in mature, hard shoots little change 

 of colour occurs unless actual charring of the tissues be effected, and there is no exuda- 

 tion on the surface. These differences are satisfactorily explained by the differences in 

 the nature of the tissues in the two cases. In the young shoots a delicate epidermis 

 provided with stomatic orifices is present covering a mass of young active hypodermal 











