

IN THE MOTOR OKGAN8 OF LEAVES. 



85 



aqueous solution of osmic acid and mounted in saturated solution of acetate of potasl 

 The superficial stratum then acquires a strong brown -madder tint, whilst the d r on. 



remains almost unaltered in colour; and sections which have been treated in this fashion 



consequently afford a very clear demonstration of the distribution of the two strata 

 throughout the pulvinus, and of the fact that the excess in thickness of the inferior ma^ 

 of parenchyma is owing to the deeper stratum of tissue on its attainh 



g a greater thick 



ness than it docs in the superior one (Plate VI, Figs. 1,2). The distinction betwe n tin 

 two strata of the parenchyma comes out less clearly, but is still quite recogni> ,hlc, m 



preparations which have been subsequently stained with picrocarmine. (Plato VI, Kig (! ) 



The structural features presented by the primary pulvinus of Minima p, i C a ai 

 thus of a nature readily to account for the occurrence of periodic movements in the pri- 

 mary petiole. The persistently soft, flexible consistence of the fibro-vaseolar bundle, its 

 vertical compression, and its relatively small bulk in relation to the masses of jmlvinar 

 parenchyma, unite to render it peculiarly liable to vertical flexion in unection with 

 alterations in the relative strength of the over- and under-lying masses of tissue, an 1 tin 

 structural peculiarities which the latter present secure that such fluctuations shall occur. The 

 characters of the cell- walls in the superficial strata in the superior and inferior parenchyma 

 imply an excess of structural strength in the former, and the greater thick] n of the deeper 

 stratum in the inferior j)arenchyma secures an excess of functional Strength and additional 

 structural weakness in it. Owing to its textural peculiarities, the inferior mass f paren- 

 chyma must necessarily be liable not only to much greater but to much more rapid 

 fluctuations in turgescence than the superior one, the greater mass of tissue rich in cho- 

 lorophyll accounting for its liability to excess in fluctuations of turgescence, and the • xeess 

 of tissue providing special facilities for filtration allowing for such fluctuations taking 

 place in large amount with great rapidity. The great r thickness in the cell-walls of th 

 outer stratum of the parenchyma lying above the fibro-vascular axis confers greater struc- 

 tural strength on it than that which is provided by tin textural features of the corre- 

 sponding stratum on the underside of the pulvinus, and the excels in thickness of the 

 deeper stratum in the mass lying beneath the axis secures greater functional strength and 

 also greater facilities for rapid filtrative loss of turgc-cence there, owing to the exec- which 

 it implies of open and elaborately pitted tissue abundantly provided with chlorophyll. 

 There is nothing to lead to the belief that the individual elements in the deeper stratum 

 of parenchyma are likely to undergo greater turgescence under solar stimulation in thai 

 portion of it underlying the fibro-vascular axis than in that which lies above the latter; 

 but the number of elements present in the former site considerably exceeds thai in the 

 latter, so that under solar stimulation a greater general rise in turgescence must occur in 

 the inferior than in the superior parenchyma as a whole. Conversely the kiss in turges- 

 cence attending removal of solar stimulation must give rise to greater total loss of turges- 

 cence in the lower than in the upper or axillary mass of parenchyma, and therefore, 

 apart from any interfering factors, solar stimulation would necessarily give rise to 

 convergence or elevation of the petiole, and absence of such stimulation to divergence or 

 depression. But, as has been already pointed out, a very powerful interfering factor is 

 provided by the variations in weight of the distal parts of the leaf. The leverage exerted 

 by the distal parts of the leaf co-operates constantly with the axillary mass of parenchyma 



akin"- for depression ; it is not, however, a constant quantity, but one which is subject 



in m 



* 



to undergo very considerable variations. It is a factor the power of which normally rises 



