IN THE MOTOR ORGANS OP LEAVES. 



65 



movements first begin to appear. The depression is determined by the accumulation 

 of parenchyma over the anterior aspect of the base of the secondary petiole, and conver- 

 gence by the extension of it along tlie course of the outer side When pulvinar paren- 

 chyma begins to accumulate beneath the base and along the axillary surface of the second- 

 ary petiole, and to extend thence over the anterior surface of the >ase of the 

 lamina and outwards along the groove which originally corresponds with the course 

 of the midrib, it tends more and more to cause elevation and divergence of the second- 

 ary petiole and expansion of the lamina; and when the tissue 1 is attained sufficient 

 structural power to resist the older masses cf tissue mnking for depression and conver- 

 gence of the secondary petiole and folding upwards of the halves of tlio lamina, a t na- 

 tion of permanent equilibrium is once more established. Until this lias occurred, how- 

 ever, movements necessarily continue to take place, because the fluctuations in turgi »nee 

 which accompany the presence or absence of solar stimulation in the younger and more 

 recently developed masses of tissue are greater than tho>e taking place in tin older one* 

 So long as this is the case, the younger tissue is relatively stronger during the tiny than it 

 is at night, and consequently it is able to effect displacements diuinally which it is incapable 

 of maintaining nocturnally. The magnitude of the movements goes on steadily increas- 

 mg so long as the differences between the functional and structural strengths of opposed 

 masses continue to increase; but, when the development of the older ma- has once 

 been completed, the movements diminish with tho «■ mtinued development f the 

 younger mass, and they ultimately disappear when the ti mo in it has overtaken that of 

 its opponent in respect to structural strength and functional weakm . At the outset the 

 tissue which makes for the diurnal position is present in very small amount as com- 

 pared with that which makes for the nocturnal one, and consequently the amount of 

 displacement which it is able to effect is very limited. Subs' juontly it increases rapidly 

 in bulk, but at the same time the permanent structural power of its opponent has also 

 undergone increase. The mass of functionally Stronger tissue increases, but the 

 strength of the structurally stronger one docs so also up to a certain point, Tho 

 increased bulk of the functionally stronger tissue enables it .to give rise to incr ised 

 displacement under the influence of solar stimulation: but so long as tho differences in 

 permanent structural strength of the two masses go on increasing, tho amount of resist- 

 ance which the structurally stronger one is capable of overcoming in the absence of 

 solar stimulation will undergo increase al-o. In consequence of this, wo find that 

 nocturnal displacement does not attain its maximum in pinmc which have just begun 

 to move, but goes on steadily increasing for a considerable time. Tho precise amount 

 of movement exhibited by any particular pinna at a given period in the course of 

 its development depends not merely on the relative bulks of the opposing masses of 

 pulvinar tissue, but on the differences which they present in regard to functional and 

 structural strength. The amount of diurnal displacement is at first small owing to 



the small bulk of the tissue giving rise to it, and the nocturnal displacement does 

 not attain its maximum at first, because the mass of ti ie which determines it goes on 

 for some time increasing in structural strength in relation to its opponent. Just before 

 the primary initiation of movement in a pinna tho supero-ezternal mass of pulvinar 

 parenchyma is already conspicuous, whilst its opponent is practically absent: in a mature 

 or motionless pinna tin condition is more or less reversed, the ma<8 of the axillary tissue 

 being somewhat in excess of that of the external one. Owing to the fact that all the 

 pinnae in a leaf are not matured simultaneously, but progressively from tho base to the 



Ann. Roy. Bot. Gard. Calcutta Yol. YI 



