THÍ CAUSES OF FLUCIDiTIOSS IS TÜEeiSCESCE
action along the piiim.Ies of tlie samo soconclaiy raoliis, coiwergenoe of the one or
It may be ef all the other seeonilarj petioles, lapijiy spreading centiifngal action alon.-
the coarse of one, two or all the other secondary mddses, and maximal action in the
prmmry pulvmiis lcadin¡r to tki fullest depression of the primary petiole. But -when
soil and air are alike damper, the extension and rapidity of movements steadily diminishand
ivhen saturation is approached, a minimum of nmvemcnt is attained except under th¡
action of other factors than mero escape of liquid from the tissues, Duiiig periods of
«cessiTC moisture the injary to the pinnule produces no immediate moyement even
locally, gradual exudation appears conspicuously at the sire of section, and this is
presently followed by slow gradaal elc-ation of the injured pinnule. In some cases no
other effect whatever follows; in others, movements are confined to the opposite pinnule,
and s„ 1 others, v „ y slow and imperfect elevation of a cer.ain n u i e r o
other prnnules rn the d.stal half or third ef the same pinna occurs. Both rate,
amount and extensren of movement most unequivocally bear an inverse relation to
condrtrons of s»l and atmosphenc moisture ; where soil-and atmospheric moisture are
present m maximal degree, the moTements following section attain their minimum in
regard to rate, amount and extension ; where soil and atmospheric moisture reach the
lowest level at which they do not lead to permanent assumption of the noctarnal
position by the pmnulcs, tho movements following section attain theü- maximal development.
The sa.me holds good ia regard to movemei^ts originating in connection with
other influences. I he movements wliich follow tho application of l.eat to the tissues
are normally more extensive than those follo^dng section, but their extent and rnpidity
are equa ly affected by the existing conditions of atmospheric and telluric moLure.
I h e application of an op.n fiame to tho tip of a distal pinnule of a distal ninnn
. . m hot dry weather followl by immediate acion, spreading rapidly ove the
entn-o leaf, and m many cases succeeded by action in many more leaves on the samj
axis both m the centripetal and centrifugal dü-cction. But, where conditions ap
proachmg satumtion prevail, similaa- ti-eatn>ent is followed by action wliich is both of
a more gradual character, and which does not exte.d beyond the pi.mules of the
dn-ectly injured pinna (Appendix 1)).
The above facts dearly show that the capacity for movements, the rate at which
they are conducted, and the extent to wliich they are propagated, bear a direct reíation
to the extent to which rapid general loss of liquid can occur; and the cuestión
arises is whether tins is to be regarded as indicating increased iiritability and
contractibikty m the protoplasts of the motor organs under the influence of conditions
favonnng general loss of liquid from the tissues, or as evidence that the movements
are dependent on losses m turgescence directly connected with the loss of liquid itself
Nom', there is no independent e^ádence that conditions of atmospheric humidity are capabb
of aetmg directly upon the irritable and contmctible functions of the protoplasts of the
motor o r p n s - n o evidence to show that the irritable and contmctible functions are
stimulated by a diy atmosphere and depressed by a moist one. J3ut there is abun
dant independent e.-idence to show that conditions of atmospheric humidity directly affect
turgescence owing to the effects wliich they produce on ti-anspiratory loss, and there
is satisfactoiy sti-uctuml evidence that the opposed masses of tissue within the motor
organs aro liable to undergo losses in turgescence in unlike degree, and at unlike
IN THE MOTOE ORGANS OF LEAVES. 101
rates, under the influence of increased tmnspiratoiy loss. The only logical conclusion
therefore which can be arrived at from the data is, that the differences in tho phenomena
of movement presenting themselves under the influence of different conditions of atmospheric
humidity are owing to con-esponding differences in the degree to which
these conditions favour the occurrence of purely phj'-sical loss of water from the tissues.
The phenomena are readily explicable on the theory that the movements have a purely
physical origin ; they remain entirely mysterious on that •\i'hich regards them as the
result of active protoplasmic contraction.
But another phenomenon frequently manifests itself in connection with the
presence of excessive atmospheric humidity which is e\-en more inexplicable on the
theory of active protoplasmic contraction, Jn cases in which not only both air and
soil arc saturated with moisture, but recent rainfall lias occurred and loaded many of
the leaves with adherent drops of water, piienomena which according to the contraction
theoiy must be regarded as indicative of special irritability of the primaiy
pulvini, frequently manifest themselves. In such cases, after ami)utation of the tip of one
of the distal pinnules, either no movement at aU occurs in the pinnules, or slow imperfect
movements occur in the injured pinnule alone, or in it and in one or two neighbouring
ones, and yet rapid and profound depression of the primary petiole takes place
shortly after tlic-injury and often before any pinnular displacement has begun to appear
(vide Appendix" A). On the theory that movements arc due to active protoplasmic
oontmction, this must be taken to indicate that excessive atmospheric humidity decreases
the iriitability and contractility of the tertiary pulvini, and increases those of the
primary one ; that is, that the same cause gives rise to diflerent effects in two parallel
sets of organs. But this is a conclusion for wliich no endence exists save tho very
phenomenon for which it has to seiTe as an explanation. But on the physical theory
there IS nothing remarkable about tlie coincidence. We know, that undi^r any circumstan<!
cs, the mass of tissue in the primaiy pulvinus -which makes for elevation of
the primary petiole bos to straggle, not only with its local opponent, but also with
tho depressant action of distal leverage; and it is clear that any accumulation of raindrops
on the pinnaj must increase the latter. But adherent raiu-drops cannot serve
to increase the resistance -which has to be overcome by the tissues in the tertiary
pulvini which make for dopr<.'ssion of the pinnules, and as a rule they must tell
dircctly against their elevation. In the primary pulvini the masses of tissue which
make for depression, and which arc structurally stronger than their opponents, are co-operated
with by the adherent rain-di-o^js in the attempt to depress the primary petiole;
in the tertiary pulvini the stmcturally feebler tissues, instead of encountering any
additional opposition under the circumstances, are aided in securing depression of the
pinnules. That is, under the circumstances, the tissues which make for the diurnal
position are relatively weakened in the primary pulvini and relatively strengthened in
the tertiary ones ; or, in other words, under the circumstances, the stability of the
equilibrium of position is decreased in the primar}' pulvini and increased in the
tertiary ones. But, if this be so, a fall in general fluid pressure incident on the
escape of liquid from tho tissues of an injured pinnule, and which is incapable of
producing any but very limited effects in the site of increased stability, may readily
induce considerable effects in that of decreased stability. So far as trant-piiutory loss
is concerned, both primary and tc:tiary pulvini are, under the circumstances, in a