

IN THE MOTOE ORGANS OF LEAVES. 



43 



influence of chloroform vapour (plate I. fig. 10). The loss in tumescence here, however, 

 is not accompanied by any such conspicuous change in colour as attends the normal 

 nocturnal loss, as the fluid which escapes from tho interior of the protoplasts into the 

 periphery of the cell-cavities is not removed and replaced by air to any appreciable 

 extent. In both cases, however, the essential phenomena are aliko and consist in a 1 



o>s 



of osmotic property in the cell-sap, dependent on depression of tho functional activity 

 of the protoplasts and determining a loss in turgescenco. Tho transitory whitening of 

 the fronds occurring under normal conditions is quite distinct from the discolouration 

 which occurs permanently in old fronds during continued periods of low temperature «»r 

 excessive dryness. The change in colour under tho latter conditions is not of the same 

 diameter, being rather a yellowing than a whitening, and is not connected with loss in 

 turgescence, but with decolourization in tho chromatophorcs, a decolonisation which is 

 apparently associated with increased formation of permanent osmotic products in the 

 cell-sap, which serve to limit the range of fluctuations in turgescence. Tho normal 

 diurnal alterations in colour, on the other hand, are essentially nyctitropic phenomena) 

 and are determined by the same primary factors as the movements which wo have 

 next to consider. 



In the case of Selaginella serpens the changes of colour are dependent on movements, 

 but these are limited to the protoplasts, and are therefore unaccompanied by any 

 appreciable alterations in size of the cells or displacement of masses of tissue. In tho 

 case of common nyctitropic movements alterations in turgescenco take place in entire 

 tissue elements or tissues, not merely in protoplasts, and servo to give rise to movements 

 as distinguished from mere alterations in bulk or consistence, because they occur in 

 unlike amount in elements or masses of elements differing from ono another both struc- 

 turally and functionally, and so arranged that any alterations in the bulk of one must tend 

 to produce corresponding alterations in the degree of resistance which it presents to the 

 action of the other. They are related to one another just as the scales in a delicate 

 balance are; and just as any alteration in the weights in the former determines displace- 

 ments in the beam to which they are attached, so do any alterations in turgescence 

 affecting one of the opposed masses of tissue to a greater extent than the other determine 

 displacements in the structures to which they are related. In the case of common pulvinatc 

 nyctitropic leaves which exhibit movements of elevation and depression, the masses of 

 tissue on the upper and under sides of the passive, flexible, unlignified vascular bundle 

 are constantly tending respectively to depress and to elevate it. So long as conditions of 

 turgescence remain unaltered, or only fluctuate in like direction and degree in both 

 simultaneously, no movement will occur ; but whenever we have either a riso or fall 

 in turgescence affecting either of them in greater degree than the other, displacements 

 proportionate to the difference in the resultant relative strength of the masses of tissue 

 must take place. The vascular bundle is in a position of unstable equilibrium, the 

 superior pulvinar pad constantly tending to depress it and the inferior one striving to 

 elevate it. The position of the bundle at any given time, in so far as local conditions 



concerned, is dependent on the relative strengths of the opposed pads, and any altera- 

 tions in their relative strengths must lead to corresponding displacement. 



The most universally diffused of all nyctitropic movements is that occurring in 

 the case of the stomata of the epidermal tissues, and as the essential features characteristic 

 of such movements as a class are in this instance presented to us in their 





are 



Ann. Roy. Bot. Gard. Calcutta Vol. VI 



