THE CAUSES OP FLUCTFATIO^'S IN TUEGESCE.YCE
under the samrj cu'ciimstances as the strength'of the mass of pnKdnar parenchyma making for
eleration increases, and hence, under uorniai circumstances, maximal elevation of the prima
r y petiole does not coincide with maximal strength of the parcnchyma making for elevation.
'J'he strength of the latter, in so far as it has to contend witli the opposing mass
of pulvinar parenchyma, liscs under the influence of solar stimulation; but this increase is
apparently more than discounted by the coincident rise in distal leverage, so that,
although its strength attains an absolute maximum under such circiimstances, the tissue is
relatively ^vcaker in respect to the total resistance which it has to overcome than it is
under the influence of continued absence of photic stimulation.
The structural peculiarities of the leaves of Mimnsa pucUaa do not, however, merely
j)ro'\'idc facilities for the occurrence of extensive movements of the primary petiole in
connection with variations in the degree of solai' stimulation <jf assiudlatory function in the
tissues, but also for the ready occurrcuce of such movoments under the influence of vanous
other conditions. 'I'he priinar\' petiole is constantly in a position of extremely unstable
equilibrium -wliich is determined by the action of the superior pulvinar parenchyma and the
leverage of the distal parts of the leaf on one hand, and of that of the inferior pulvinar
jwreiiciiyma on the other. Any alterations in distal leverage or in the relative strengths
of tJie opposing masses of parenchj-ma must inevitably lead to movemoiits of the petiole
in one dii'cction or anotlier. Any increase in distal leverage must tend to induce
depression or divergence of the petiole, not merely in connection with coincident compi'ossion
of the tissue of the inferior pulvinar parenchyma, but because of the increased
iiltrative loss of tiu-gescence wliich this impUes; and any decreased leverage to corresponding
elevation up to the point at which the colls of the inferior parenchyma have
attained the gr-eatest degi-ee of turgescent increase in capacity of which they are capable.
But vaiiatious in distal levcmge do not take place solely in connection with variations
in the conditions of assimilatory activity of the tissues. Any addition of weight to the
tlistal portions of the leaf whether of intiinsic or extiinsic origin, or any additional
pressure on the upper surface of the petiole, imply correspouding increments of leverage,
and hcnce we find that, especially when these are developed suddenly, they are liable to
be followed b y depression of the leaf. On th3 other hand, we liave already seen how
rapid loss of weight in the distal parts of the leaves, as the result of excessive
eva])oration there, may' lead to extensive movements of elevatiftn.
The tis ues of the primary pxilvinus not only provide faciliiies for the occun-ence
of extensive movements in the primary petioles, but they are fni-thcr especially adapted to
permit of tliese movements being executed with gi-eat rapidity undur certain circimistances.
The fibro-vascular bmidle is permanently weak, soft and flexible, and the mass of pulvinar
parenchyma which underhes it pre>ent.s excessive stmctui-al facilities for rapid filtrative
redistribution of lai'ge masses of fluid -within its deeper stratum, with its thick-walled
and elaborately pitted cells and great system of intercellular spaces. We have already
seen that rate of movement is directly related to the degree in which the pulvinar tissues
present structural peculiarities favoming rapiti alterations in tm-geacence, and that the
tissues in the primary pulvini (and specially in those poiticns of parenchyma underlying
the fibro-vascular axis of the primary pulvini of Mimom pudiao) are endowed with such
peculiarities in exceptionally high degree. The superior mass of pulvinar parenchyma,
aided by the leverage of the distal parts of the leaf, is constantly making strongly for
depression of the primary petiole. Elevation is only maintained by means of active
IN' THE MOTOU OliGANS OP LEAVES. 87
i of the inferior pareiichynna sustained in spite of excessive local facilities for
rapid filtrative escape of fluid and the combined compressant action of the superior parciich}
raa and distal leverage, so that anything giving rise to sudden loss of osmotic property
in the cell-sap, to sudden diminution in the supply of fluid to satisfy the osmotic
capacity of the latter, or to sudden increase in the conditions of local pressure affecting
filtration nmst inevitably lead to sudden movements. Active turgescence of the excessively
porous tissue of the deeper stratum of pulvinar parenchyma can only be maintained
as the result of the continuous addition of highly osmotic and unstable assimilatory products
to the cell-sap with coincident abundant supply of fluid to make good the constant
filtrative loss whicli is going on in the tissxre; and it is only under conditions of high
turgescence that the inferior mass of pulvinar parenchyma is able to overcome the conjoint
action of the superior mass and of distal leverage so long as the latter is maintained
at a high level. The superior mass of pai-enchyma is not devoid tjf tissue presenting
special facilities for rapid redistribiition of fluid, but it contains much less of such
tissue than the superior one docs, and hence ai\y conditions determining general loss in
turgescence must produce greater and more rapid weakening of the inferior than of the
superior mass. Under ordinary circumstances the loss of strength in the superior mass
is thus concealed by the greater loss taking place in the inferior onu, and it is only
where the action of any agent inducing a fall in turgescence is carefully limited to the;
superior mass that we have any indices to its action upon it {oide Appendix 13).
The structural pcciiliaaities permitting of the occurrence of rapid loss in turgescence in
the inferior mass of puhiuar tissue also pei-mit of rapid gain, processes of rapid endosmose
being provided for by the excessively porous character of the cell-walls and the
abimdant chlorophyll contents of the deeper stratum of tissue. l"he process of elevation is,
of course, never so rapid as that of depression, because it is resisted by distal leverage,
and is mainly carried out by osmosis in. place of filtration ; but in spite of this it is
veiy much more rapidly conductcd than it is in cases whore the pidvinar tissues do not
provide the same facilities for rapid absorjition of fluid, and under favoiirable circrunstances
of illumination aud water-supply, its progress is readily visible to the unaided
eye {vide Appendix J3).
The stnictural peculiarities of the inferior masses of puhinar parenchyma in the
primary pulvini of 3Iimo8a pudica micquivocally provide facilities for the occuri-ence of sudden
loss of tm-gescence in the tissue, but the movements in the primary petioles arc not
invariably suddenly execxited. The capacity for the occurrence of sud<len movements is
dependent on the presence of certain structural peculiarities facilitating I'apid movements
of fluid in the tissues ; but the actual occiu-rence of sudden movements, even in cases
where such prculiaiitios are present in very In'gh degree, depends on the sudden incidence
of conditions making for the redistribution of fluid. Hence in Mimosa pudica
any gradual diminution in water-supply only causes gradual movements in the primary petiole
in spite of the esccssivo textm'al facilities for escajro of fluid from the tissues of their
motor organs ; aud, further than tlds, the effects of gradual local alterations in turgescence
in the latter may be more than discounted by coincident alterations iu distal
leverage, so that elevation in place of depression may occur. It is only under such circumstances,
however, that movements of this reversed character are present iu connection
with loss of strength in the inferior as compared with the superior pulvinar parenchyma.
They never occur, in connection with conditions implying local increase iu