

MOTOR OBGA^S OF LEAVES. 



49 



arrangements of this type. At a very early stage of development the two halves of 

 the lamina have their upper surfaces closely applied to one another, whilst in the mature 

 lamina they are permanently expanded in the same plane; but an intermediate stage 

 exists during which periodic diurnal and nocturnal movements of alternate expansion 

 and approximation of the opposite halves take place. Here there is no appearance of 

 opposing masses of active tissue telling on an intermediate passive one, but the oppos- 

 ing tissues which deteimine movement are in direct relation to one another. It is the 

 growth of parenchyma over the line of the midrib which ultimately gives rise to perma- 

 nent expansion of the lamina by forcing out the divergent woody bundles of tli. 

 veins from their original direction, and which in the intermediate stage rives rise to 



.->- e> 



gr< •< • 



periodic nyctitropic movements of the two halves in relation to variations in the de 

 of turgescence in the. growing tissue under the influence of sunlight and darkness. 

 All that is necessary in order to the occurrence of nyctitropic movements is the 

 presence of opposing tissue elements or masses of tissue, differing from one another in 



structure and function, and undergoing unlike fluctuations in turgescence under tin 

 influence of particular conditions. 



The differences distinguishing the masses of tissue which make respectively for tin 

 diurnal and nocturnal position of entire leaves or large parts of leaves are of the same 

 nature as those which we have already seen to distinguish the guard-cells and tin 

 common epidermal elements. The tissues making for the diurnal position aie rela- 

 tively late in development and rich in protoplasm, and specially in chlorophyll, as com- 

 pared with those which make for the nocturnal position. But relative youth and 

 abundance of protoplasm & priori imply relative structural weakuess and functional 

 strength; and the presence of chlorophyll implies a special assimilatory exercise of the 

 latter - under the stimulant influence of sunlight. Under the latter a greater rise in 

 turgescence will thus take place in the masses of tissue which make for the diurnal 

 position than in those making for the nocturnal one. During exposure to sunlight 

 there is a general rise in turgescence, but this is not uniformly distributed, but takes 

 place to a greater extent in one of the opposing masses of tissue than in the other; 

 and the foimer thus becoming temporarily the stronger is able to determine displace- 

 ment of the parts to which it is opposed. Passive structural strength must, of course, 



remain unaffected by exposure to light or darkness, but the strength arising from 

 functional sources is not constantly alike, but fluctuates with the degree of turgescence 

 present at any given time. Any tissue in which an excess of turgescence arises in 

 connection with exposure to certain conditions must become relatively stronger than a 

 tissue in which there is not an equal rise, the difference in strength being directly 

 related to the difference in degree of turgescence. Under the influence of solar stimu- 

 lation we have greater assimilatory activity in the tissues making for the diurnal than 

 for the nocturnal position. But turgescence being directly related to assimilatory 

 activity, this implies a corresponding excess in strength and a proportionate rise in 

 capacity for resisting the pressure of opposing tissues, and consequently displacement of 

 the latter occurs. 



Apart from solar stimulation and general transpiratory loss, the position of the 

 various parts of nyctitropic leaves will be determined by the relative structural 

 strength and permanent functional activity of the opposed tissues in the motor 

 apparatus. On the incidence of sunshine, structural strength remains' unchanged, but 

 an excess of functional strength— an excess of turgescence— arising in one of the opposed 

 tissues, displacement must occur in direct proportion to the excess. The amount of 



Axn. Roy. Bor. Gard. Calcutta Vol. VI. 



rv.o. Bot. Garden 



1909 



