HO THE CAUSES OF FLUCTUATIONS IN TUKGESCENCE 



the system of sieve-tubes in the bast, as Haberlandt maintains, this may well give 

 rise to sudden increase in the amount of filtration occurring from the actively tumes- 

 cent tissues of the motor organs. When the pulvinar tissues which make for the diurnal 

 position afford much greater facilities for nitration than those which make for the 

 nocturnal one, they must, under the circumstances, rapidly become relatively weakened, 

 and consequently rapid movements of elevation and convergence of the pinnules 

 must be liable to occur. The liability to the occurrence of rapid movement must vary 

 directly with the degree to which the opposing pulvinar masses differ from one another 



jet to the filtrative facilities which they present, as it is on the presence of 

 such differences that alterations in their relative strength are, under such circum- 

 stances, determined, and hence movements are much less constant and conspicuous in 

 the secondary rachises than they are in the pinnules. But, so long as transpiratory 

 loss remains low, such filtrative escape does not imply absolute loss of liquid; it 

 merely implies the displacement of a certain amount of liquid from the interior of 

 the actively tumescent elements of the motor organs. A local supply of liquid will 



m resp 



b 



thus remain available for expenditure in the re-establishment of active turgescence, 

 the precise amount varying, of course, with the amount of reserve liquid originally 

 present and with the activity of transpiratory loss for the time being. Consequently, 

 under the influence of continued exposure to photic stimulation and low transpiratory 

 loss, turgescence must tend to be more or less completely re-established, seeing that 

 nothing has happened to interfere with the assimilatory activity of the tissues of the 

 motor organs, and that a supply of water is present to meet the increased osmotic 

 capacity of the cell-sap connected with continued assimilation. But as the masses 

 of pulvinar tissue which afford the major facilities for filtrative loss are also those 

 which under the influence of light undergo the greatest increase in turgescence, con- 

 tinued exposure must lead to their gradually regaining their relative strength to a greater 

 or less degree, and with this a resumption of the diurnal position must be correspondingly 

 established according to the available supply of water. A new position of unstable 

 equilibrium is thus attained, and may remain unaltered for a considerable period so long as 

 external conditions remain the same. But if the leaves be now suddenly exposed to the 

 direct rays of the sun, they are placed under conditions which imply increased transpira- 

 tory loss. If the increase be a very limited but progressive one, as it is on very damp 

 mornings or when the sunshine is feeble, the effects which it will produce will be 

 delayed, and when established will be of a slowly progressive character : the position 

 which had arisen under the previous conditions will no longer be maintained, but the 

 departure from it will not begin for some time and, when fairly established, will be 

 of an insensibly progressive character, because the transpiratory loss which causes it 

 is a very gradual one. In any case the drain will tell most heavily on those tissues 

 which present the greatest structural facilities for escape of fluid, and, consequently, the 

 initial effect of exposure to direct sunsliine is the establishment of movements towards 

 the nocturnal position. But where exposure implies the establishment of sudden and 

 great increase in transpiratory loss, the movements may no longer be of a slow and 

 insensibly progressive character, but may be conducted as rapidly, or almost as rapidly, 

 as those attending the original separation of the leaf from the axis. The sudden and 

 great increase in transpiratory loss here plays the same part as the active discharge 

 of fluid from the detached petioles in giving rise to a considerable fall in liquid 

 tension, and, with tliis, sudden and excessive drain upon those masses of pulvinar tissues 

















