THE CAUSES OF FLUCTUATIONS IM TUIiGFSCKKCJi
SO long OS Iho tissues mlam tlmir vaality, but tins is by no means the rule witli
olher „yctib-opio uoTeinent. In oeitaiu »so, ,ve do find the latter i-etaini,..- a
MUitorm cliaraeler throaghoiit, but in most oases tlwy undergo eonsidorablo modii,
cation in regard to their- preeiso nature, and espedally in regard to their extent
as tune goes on, and ni manj' cases ire find them almost or. entirely absent
during eonsiJerable periods in the latter part of the life oi the leaves in which
they once were oonspieuously present. As we hayo already seen, they only b e a n to
inaniiest themselves atter the evolution of the leaves has reached a eeitaiii poiirt and
subsequently we find tl.em, as a rule, rapidly increasing in amount, attaining a maximum,
and then declining from this to a greater .or less extent. The various parts of the leaf
are primarily in a eondition of stable equilibrium: this is succeeded by one of unstable
cqmlibrium which gradually attains a maximmn and then declines from tiris towards,
or absolutely to, renewed stability. The position of the various parts of any leaf
is nccessaiily determined by the structural pceuliarities and degree of turgesccnoe wliieh
they possess. Structmal peculiarities arc necessarily liable to modification with increasing
age, but at any particular time they may bo regarded as fixed factors. This is not^
however, the case in regard to c.mditions of tnrgescenee which are liable to undergo
conslant iiietiiations in connection with variations in protoplasmic stimulation and activity,
and fluctuations in the relations between root-sapply and transpiratoiy loss. Where
all the tissues are subject to like fluctuations in tm-gessence, mere alterations in their
consistence culminating in one direction in free excretion of water, and in the other
in general wilting will oeem-, but no definite movements will present themselves. It
is only when special masses of tissue, differing from one another in their capacity for,
and liability to, fluctuation in turgeseence are present that orderly, delinite movement.s
will occur, and then only in association with cei-tain structural peculiarities. Nyctitrojuc
movements, therefore, otdy present thcm.selves after leaves have attained a certain
stage of development, and in connection with the evolution of specially .situated masses
of tissue, so disposed as to provide an active oppositimi between structural and functional
factors, and it is only so long and so far as the cUfTerenees in structural and functional
properties of these masses persist that inovements vdll occur. The relative youth of the
masses of ti.ssue making for the dim-nol position at first provides that the diScrence
•shall be very considerable, and movements are accordingly very conspicuous for a time,
but as in most cases, at all events, this difference natnrally tends to diminish witli
the increase in structural strength and expenditure of active protoplasm of the younger
tiissnes, a condition of stable equilibrium is once more approached or actually attained
with a corresponding diminution or abolition of movement.
Great differences oceiir in regard to the i-ate at which the movements, whether
of a normal periodic character or arising as the result of extrinsic agencies affecting
filtration or water-supply, are carried out in diflerent Icave.s, the rate bearmg no
definite relation to the magnitude of movement, but being related to structm-al features
afiicting the redistribution of fluid throngbout the tissues The movements are
primarily duo to fluctuations in the osmotic properties of the eell-sap, to fluctuations
in external pre.ssure, or to fluctuations in water-supply, giving rise to coiTcsponcUng
alterations in the degree of turgesecnee of the various parts of the motor organs; but
the rate at wdiieh these alterations can be effected must necessarily be influenced by
the straetural features of the wafls of the colls entering into the constitution of the
THE MOTOE ORGA-'iS OP LE-iVES. 57
tissues as favouring or obstructing tlio access or escape of fluid. Taking any highly
turgid cell, in which a great fall in the osmotic capacity of the ceU-sap sets in, a
proportionate loss in turgeseence must ultimately ensue owing to the flltrative dischnwe
caused by the elastic recoil of the cell-walls; but the rate at which the discharge is
eondnctcd mu.st be regulated by the facilities which the latter provides for flltration
and by the degree to which ready accommodation for the escaping fluid is present
rins being so, the ™te at which movements oecur ought to bear a recognisable relatio.i
to be degree of pitting of the cell-walls and to the openness of the piilviii.r parenchyma;
and direct evidence that this is actually the ease is not wanting. The movements whieh
are executed by the pinn® of Ormi« alata are certainly, in so far as maonitnde is
concerned, fairiy comparable with those occurring in any part of the leaves of
but the rate at wHch they are conducted in the two eases is very different
the movements in the former being very slowly carried out as compared with those
m the latter. Coinciding with tliis difference in rate of movement, we find very
conspicuous differences in the structural details of the pulvini; the pulvinar tis.sue in
Casern alata being throughout dense and composed of cells wliieh are provided with
only a limited number of small pits, wdiilst in Mimom pudic, the deeper strata of the
tissue arc net only very open in texture, but arc composed of dements characterised
by an abundance of huge pits. (Plate VI, Figs. 3, 4, Vl.j
The facilities for filtration must evidently bear a direet relation to the extent to which
the cell-walls are pitted; and where e-xtensive pitting is combined with very considerable
thickness and strength of the eelbwalls elsewhere, as it is in the primary pulvmi of
J / » » « hltratiee facilities naturally attain a maximum (Plate VI, p t s 3 4
12; Plate MI, Fig. ,5). In the case of the movements occm-ring i„ tlio L v e s J
i « ™ « and CasA sumalrana, we also find very clear evidences of a correspondence
betiveen rate of movement and te.xtnral facilities for filtration The move
nients in Z » » ™ are relatively rapid, those in Oa^sia mnatra,a comparatively
euuoouus'lsyi r pi-tttetdT m Jth e :f ormer than™ in the latter ease (Plate VII, Fi-g«s.« h2 , .™4)r.e conspii
fiItraL"f°'T.'' '»"' "»""«¡"rf «ith greater difficulty where
s a b ^ v t "" """ "" ""I » » » l u o n t l y [hat the
. abdity of any position maintained as the result of the active targe cence of a mass L r ; : " "
to find ; "" 8™'. ^ve ought
n e ^ i l T - ° r - conditions which are
J d not, therefore, surprise us to find conspicuous movements occinring fa the 1 ave
must be frequently repeated. The d i s t a i W e to whieh th leaves a
«.0 .mount of fluid a c t L l y - c h a r ^ r l r S f t f ^ e s ' ^ : ^ X t ^
Ass. Koi. BoT. G»,d. CircoTVi Tel., VI.