no THE CAUSES OP PLUCTUATJOîiS IN TOEGESCEXCE
tho system of sieve-tubes iu the "bast, as Ilabcrlanclt maintains, this may well give
rise to sudden increase in the amount of filtration occurring from the aetively turgescent
tissues of the motor organs. When the pulvinar tissues which make for the diunial
position afford much greater facilities for filtration tiian those which make for tlie
nocturnal one, thoy must, under tho circumstances, rapidly become relatively weakened,
and consequently rapid moTcments of elevation and convergence of the pinnules
jnust be hable to occur-. The liability to the occurrence of rapid movement must vary
directly with the degree to which the opposing puh-inar masses differ from one another
in respect to the filtrative facilities M'hich they present, as it is on the presence of
such diffei-ences that alterations in their relative strength are, midor such chcumstances,
determined, and îience movements are much less constant and conspicuous in
the secondaiy rachiscs than they are iu the pinnules. But, so long as transijii-atory
loss romanis low, such filtrative escape does not imply absolute loss of liquid; it
merely implies the displacement of a certain amount of hquid from the interior of
tho acLively turgescent elements of the motor organs. A local supply of hquid will
thus remain available for expenditm-e in the re-estabhshment of active turgescence,
the precise amount Tar}ing, cf eom-se, the amount of resen'e liquid originally
present and with the activity of transpiratory loss for the time being. Consequently,
under the influence of continued exposure to 2>hotic stiiimlation and low tronsphatory
loss, turgescence must tend to be more or le^s completely re-estabhshed, seeing that
iiothiug has happened to interfere with the assimilatory activity of the tissues of the
motor organs, and that a supply of water is present to meet the increased osmotic
c:apacity of the cell-sap connected with continued assimilation. But as the masses
of puK-inar tissue which afford tho major facilities for filtrative loss are also those
which under the influence of hght undergo the greatest increase in tm'gescence, continued
exposure must lead to thch gradually regaining their relative strength to a greater
or less degi-ee, and with this a resumjition of the diurnal position must bo correspondingly
established accorcHng to the available supply of water. A new position of unstable
equilibrium is thus attamed, and may remain unaltered for a considerable period so long as
external conditions remain tho same. But if the leaves be now suddenly exposed to the
dii-cct rays of tiie sun, they arc placed under conditions wliich imply increased transpiratory
loss. If tho increase be a very limited but progressive one, as it is on very damp
mornings or when tho sunshine is f^jeble, the effects which it will produce will be
delayed, and when established will be of a slowly progressive character : the position
which had arisen under the previous conditions will no longer be maintained, but the
depai-ture fi-om it will not begin for some time and, when fairly established, will be
of an insensibly progressive character, because the transpiratory loss which causes it
is a very gradual one. In any caso the drain will tell most heavily on those tissues
which present the greatest stmctural facilities for escape of fluid, and, consequently, the
initial effect of exposure to direct sunsliine is the establishment of movements towards
the nocturnal position. But where exposure imphes the establishment of sudden and
great increase in transpirutory loss, the movements may no longer be of a slow and
insensibly progi-essive character, but may be conducted as rapidly, or almost as rapidly,
as those attending the original separation of the leaf from the axis. The sudden and
great increase in transpiratory loss here plays the same part as tlie active discharge
of fluid from the detached petioles in giving rise to a considerable fall in liquid
tension, and, with tliis, sudden and excessive drain upon those masses of pulvinar tissues
I.v THE MOTOE OEGA^'S OF LEAVES. lU
wliich afford special facihties for filtrative escape of fluid. Under continued exposure,
however, an assimilatory struggle for the remaining supply of water is once more established,
and the tissues making for the diurnal position once more undergo a relative increase
in strength. Tliis for some time enables them to give rise to a partial resumption of the
diurnal position even under the new conditions of transpiratory loss; but, as the latter
contiimes, they agam become wealtened, and the various parts of the leaf gradually and
finally assume the positions corresjionding to the passive structural strength of tho various
tissues entering into theii- composition.
In the afternoon of the same day, supposing that it has been one of continuous
sunsliine, and that no special interfering factors have come into play, the conditions both
of the tissues themselves and of their environment are very different from -wliat they
were in the early mi>rDing. The tissues have for so long been exposed to active transpiratory
loss as to ensure a great diminution, if not a total expenditure, of any surplus
of water-content beyond the amount necessary to meet the immediate demands of the
active elements, and atmospheric conditions are such as to imply much more activo
U-anspiratory loss than that which was present in tho morning. Under the circumstanccs,
it is only natural that aay phenomena of movement dependent on alterations in the
relations of supply and loss of water should show certain modifications in character, and
that these moditications should be of the nature of those wliich are actually present.
The supply of reserve-water within the water-conducting system being greatly decreased
or practically exhausted, and transpii'atoiy loss being greatly increased, the maintenance
of the diurnal position is sccui-ed only by means of immediate expenditui-e of the water
furnished by the stream traversing the water-conducting system from the roots • and
hence separation from the axis implies an immediate and very considerable drain upou
the active tissues. Tliis secures proportionately rapid and complete action in tho
pinnules and considerable ni lOvement in the secondary rachises. Uiidoi' such circumstances
the recovery of the diurnal poshion is naturally less complete, exposure to direct sunshine
is followed by more rapid movements and more frequently by abrupt, spreading
movements, and the secondaiy assumption of the nocturnal position is not only much
more complete than it was in tho morning, but is also final aud is never succeeded by
any symptoms of secondary recovery.
In dealing with these phenomena of movements in leaves and shoots when separated
from^ the axis and deprived of root-supply of water, it has been pointed out that exposure
to dii-ect sunshine does not nm-mally give rise to abrupt, but only to insensibly progressive
movements. The exposure necessarily implies just as much increased transpii-atoiy lo.=.s
as it does in the case of detached leaves, and yet the occm-rence of sudden movements is
quite exceptional in place of being a frequent or, under certain circumstances, a constant
event, as it is in tho latter. This is readily explicable on account of the fact that leaves
whilst thoy are attached to the axis are not solely dependent on inherent water-supply,
but are permanently practically in a condition parallel to detached leaves containing a
largo amount of reserve-water. Tho continuous supply of water- ascending from the
roots replaces the local reserve of water in detached leaves in which only slow insensibly
progi-essive movements occur. In neither case is there any normal tendency to
rapid, sudden movements, because the sudden increase iu transpiratory loss is to a great
extent equilibrated by the presence of an ample supply of water. The sudden increase
renders the maintenance of the maximal diurnal position impossible, but the loss iu