ger 



54 THE CAUSES OF FLUCTUATIONS IN TUBGESCENCE 



of the upper or axillary pad during the later stages of the evolution of the leaf merely 

 serves to aid the increasing leverage of distal parts in determining a gradual and pro- 

 gressive increase in divergence from the line of the axis. In such cases the arrangement 

 and character of the vascular bundles of the pulvinus are such that mere fluctuations in 

 turgidity are unable to give rise to corresponding displacements of the petiole, and it is 

 only as the result of continued growth that the degree of divergence is determined. 



There is thus both dynamical and structural evidence that the masses of tissue in 

 the motor organs which make for the diurnal position are younger, or contain a la 

 number of young elements, than those which make for the nocturnal one; and thi 

 itself almost necessarily implies that, as a whole, they are structurally weaker and 

 functionally stronger than the latter are. Taking any ordinary unit of vegetable tissue, 

 there can be no doubt that, as a rule, the proportion of active protoplasm in it tends 

 to diminish, and that of formed material to increase, with increased age. The formed 

 material may take the shape of secretions of various kinds, or may enter into the con- 

 stitution of the cell-wall. In any case there is a tendency to diminution in the relative 

 amount of protoplasm, and, in cases where continued additions are made to the cell-wall, 

 these imply increased structural strength, and hence, speaking generally, the larger the 

 number of young elements in any tissue, the greater will its functional 'power be; and 

 the larger the number of old elements, the greater its structural power. It follows from 

 this that m pulvini in which we have to deal with opposing masses of tissue differing 

 from one another in regard to the period during which active growth continues, we are 

 necessarily dealing with an opposition of functional and structural strength, the less 

 mature tissue being functionally, and the more mature structurally the stronger. But 

 the effect produced by exposure to stimulation must be proportionate to the amount of 

 material to be stimulated; and hence, under exposure to any conditions implying protoplas- 

 mic stimulation the functionally stronger tissue will undergo a greater increase in 



iu "T, r Stmctura % 8t ™^ one, and where the increased activity leads to 



sed tumescence as that induced by solar stimulation unequivocally doe* it will 











nclergo a corresponding increase in relative strength. Under sola, 



takes place, but tl 



ilation 



bein. erea w ^ * «. - ^ " 0t eqUa " y distributcd i ■*. *ho i»««* 



nd°ena 1 it to I J ^ *"* ** fact ** V * h a te '»l«>-v advantage 



ie hfp si L ~f 7T / n *"■ 8trUCtUrall >- » troD ^ Z Which <W 



, . pC8ltlon a P art from solar stimulation. The cord-like imn«» t of 

 vascular tissue, and the absence of I" " arrangement of 





jmific 



fibr< i 



gnmcation m typical pulvini, provide facilities for 





the action of the opposed masses of parenchyma i^al "ani r 7 



consequently arise in connection Jth fluctuations i n Z ^n e ^ 

 incidence or removal of solar stimulation * 





coinciding with tl 



The masses of tissue which make for the diurnil n«™^ . , ■ u 



>rotonlasm. m H. fU^. ^_^ ..._, *' paSltl0n are not ™** l T nch 



in protoplasm, and, therefore, endowed witl 



i greater general functional activity than 



actua Uy present, i, „„ conclusive evidence ef increased tumescence a i,Tv » , T am « m ^ <«'**• ™» if itbe 

 a cond lllon 0( more staUe ibr . posset undec the fl 7 '1^ """ tbe '"^ h "° '' aSSed '^ 



of the nocturnal po siti „u iMows ie!ectire , ™°* ' » £*» f° »««■« ol lig M, and the fact that .„ assumption 



c-nnected ™ h aecreaacd tumescence in the Ifor r S an S I the , T* ^^"^ " sh ^ ** » " 



the motor apparatus can he made the .ubieot ot TL i, °™ ° f the S,0mala in ■«* the condition of 



precise!, ,he same conditions a, thev do £^^J*~T* *** itt " hic " *° —- ts occur under 

 to decreased tumescence in the active elcn,e»T ' " d ° n0t &ai d **°™ JP™« rise to increased but 



