126 CUNNINGHAM ON FLUCTUATIONS IN TUMESCENCE. 



6. Strong elevation, followed by depression. 



7. Ditto, ditto. 



8. Ditto, ditto. 



9. Depression, 



1 0. Ditto. 



11. Strong elevation, action in the pinnules; nothing more. 



12. Depression. 



13. Strong elevation, followed by depression. 



14. Elevation; a pause; action in the pinnules; nothing more. 



15. Ditto; depression. 



The movements of elevation of the primary petiole occurring in such cases are, as a rule, followed 

 by depression, because it is only initially that the loss in turgescence following the insolation is confined 

 to the superior mass of pulvinar parenchyma, and because, when it has once extended so as to affect 

 the inferior mass, the persistent excess in structural strength of its opponent and the leverage of the 

 distal parts of the leaf come into play as they ordinarily do. Under circumstances implying general 

 loss in turgescence the movements of the primary petiole are always movements of depression, because 

 the structural features of the inferior mass of pulvinar parenchyma are such as to render it more liable 

 to rapid losses in turgescence, and because it has constantly to struggle against, in place of being 

 aided by, distal leverage in giving rise to the displacement which it makes for. That sudden 

 movements of elevation should occur at all is, however, sufficient to show how unfounded is Sachs' 

 statement that "die Unterseite allein ist also reizbar, das Parenchym der Oberseite ist nur Hilfsorgan 

 bei der Bewegung."* There is absolutely no evidence tc show that the protoplasts of the superior 

 parenchyma are less contractile than those of the inferior parenchyma. The two masses of tissue 

 differ from one another both structurally and functionally, but the functional differences are connected 

 not with contractile, but with assimilatory property. The superior parenchyma is distinguished by excess 

 in structural strength and the inferior one by excess in assimilatory activity, and it is the presence of 

 these differences, combined with the fact that distal leverage co-operates with the superior and directly 

 opposes the inferior parenchyma in effecting the displacements which they make for, that determines the 

 ordinary character of the movements of the primary petiole. It is the presence of these differences 

 and the action of distal leverage, and not the presence of any specially contractile protoplasts in the 

 inferior parenchyma, that accounts for the fact that sudden movements of depression of the primary 

 petiole are so common, and sudden movements of elevation quite exceptional. 



The following experimental data illustrate the effect of variations in conditions of distal leverage 

 in modifying the character of the effects following amputation of distal portions of leaves attached to 

 growing plants: 



Experiment I. — June 14M, 3 p.m. Rain falling heavily; pinnules as a rule elevated or partly elevattd 

 owing to recurrent agitation by rain-drops; primary petioles Jairly well elevated in almost ail cases. — Amputa- 

 tion of the tips of terminal pinnules in leaves in which they were only partially elevated was, in all 

 cases save two, immediately succeeded by deep depression of the piimary petiole. Of the two 

 exceptional leaves one was supported by a neighbouring one, and the other was free of any adherent 

 drops of rain. The action in the pinnules was in all cases very slow and confined to the injured 

 pinna, and in most cases was very imperfectly carried out. At 6 p.m. the leaves were all in full 

 nocturnal position ; but although the evening was absolutely still and excessively humid, maximal 

 depression of the primary petioles in almost all cases succeeded the slightest contact impulses. The 

 influence of distal leverage in assisting to determine petiolar depression comes out very clearly in 

 regard to both periods of experiment. In the former one atmospheric and telluric conditions were 

 such as to be manifestly unfavourable to the occurrence of rapid losses in turgescence, as the behaviour 

 of the pinnules very clearly shows ; and yet petiolar movements of great magnitude occurred with 

 excessive frequency because of the abnormal excess in distal leverage provided by the adherent drops of 

 rain. In the Becond period also, facilities for transpiratory loss were minimal, and root-supply was 



* Vorlesung, XXXVII, s. 7.92. 







