S I
116 IHE CAUSES OP rLUCIFATIONS IS TÜRGESCE.XGE
tall to n low level and tcmporatm-o is high, it is very dilScult to seoiire sti-ictlv
localiied pmiiular moTomeiits, or oven slrietly localised moTOmnts in the prinmiy
petioles; action almost invariably being diffused along the comsB of the rnchis to some
extent in the case of pinnular "stimulation," and sudden depression of the primary
petiole being followed by piunular action in one or more of the secendary rachises,
and in some caies by varying degree« of eoi.rergenco of tho latter themselves. Tie
diffusion of movement subsequent to sudden depression of the primary petiole is no
doubt mainly oiving to the sudden iqjward pressure to which the pinnules are eiposed
from the air during the course of tho rapid descent of the leaf, and to the fact
that, in the position of ful! depression, leverage comes in to make for convergence of
tlie secondary rachises. These agencie, are incapable of giving rise to any appreciable
effects under the conditions of relatively stable equilibrium present in connection with
excessive tclbmo aud atmospheric humidity, btit are liable to produce some effect under
coltditions ill which the drain upon the actively turgescent tissues of tho motor organs
is higher. In any case, contact-" stinmlation " is a factor affecting processes of local
tiltiatiou, but it will naturally give rise to different ultimate effects accoi-ding to the
degree to wdiich the turgesceut tissues are already exposed to active (h-ain of liquid.
The upward pressure of the air on the rapidly descending laminar surfaces in cases
where the primary mevement is one of petielar dein-ession, replaces the upward pressure
winch any primarily displaced pinnule normally exerts on its distally situated neighbour.
Just as primary petiolar depression is followed hy an entire absence of pinnular
movements under conditions of high humicUty, so in many cases under smiilar
circmnstances the elevating action of the primarily displaced pinnule is insufficient to
give rise to anythmg save a certain amount of flexion in the lamina on which it
acts, without any evidence of any alteration in pulvinar tm-geseence.
The mutual relations of the laminar surfaces of adjacent pinnules is amply sufficient
to account for diffusion of movement as a mere result of diffusion of conditions
favouring filtration, so long as tlie movements follow a centrifugal direction only.
The successive laminie in their mutual relations precisely correspond to the successive
(^ards or blocks of wood in the old familiar childish game of " J a c k going to market."
Primaiy displacement of one at one end of the series is all that is necessary to
seoure the propagation of movement along the entire line. If the anterior cards or
blocks be firmly supported,—if they be
impulse from their posti-riorly situated
give rise to displacement under conditions
iny appreciable effect. The prepagation
in a cendition of stable equilibrium—an
leighboure, which would amply suffice to
of unstable equilibrium, will fail to produce
of movement ahmg the series—the "irrita
b i l i t y " of the apparatus—rises with the instability of equilibrium just as it does ...
the case of the pinnules. Whilst this is so in regard to centrifugal diffusion of
movements along the course of the secondary rachis, centripetal propagation cannot
he so smiply acconnted for; as there is no satisfactory evidence that, when in full
diurnal position, each successive lamina exerts any considerable active downward pressure
on the upper suiface of the one behind it. It is possible, however, that such pressure
may, in certain cases at all events, be present in some degree, and in so far as it
is, its sudden removal on the elevation of the depressant pirmulo must, of course, be
equivalent to a rise in the relative strength of the inferior pulvinar parenchyma of
the depressed pinnule, and consequently a faetor calculated to give rise to disturbance
in the equilibrium of position in the latter. The fact, however, that movement of
1:! • I
rjr THE MOTOR OBGANS OP LEAVES.
i i r
the pimmles oiiginating in connection with contact is much less liable to be diffused
to any considerable extent eentiipetally than centrifugally, seem, to indicate that any
such removal of pressure ,s a factor of no considerable importance; and it annoa/s
probable that the spread of centrifugal movement is aided by, and that of cent r e a
movement mainly dependent on, the action of an agency of a perfectly distinct
So long as transpiratory loss is low and root-supply of water is abundant, tho
prnnary filtrative discharge of fluid incident Oil the J ocal inci'easc in pressure caused
by contact implies a mere local redistribution of water and no diminution in the
stock available for the maintenance of turgeseence. Tho mechanical disturbance ensures
an increased activity of filtrative discharge and coiTcsponding decrease in turgeseence
ni certam masses of tissue; but it does not interfere otherwise with the assiinilatory
aetivity of the protcplasts on which the osmotic capacity of the cell-sap and censoqnently
t i e turgesceut capacity of tho cells depends. On the cessation of the
hltrativc disturbance, they will, therefore, at once begin to regain their normal slate of
turgcscenee, and, owiug to the abimdance of water which is available, they will readily
ba able to do so without in any way interfering with the tm-gescen^e of n e i g h b „ „ i ní
elements. But where conditions of humidity are such that turgeseence is only
aamtamed as the result of an exact equilibration between root-supply and transpiratory
OSS tins IS no longer t ie case. A certam amount of the „ t e r fischarged Tem
he turgid elements of the active tissues, all of it, at all events, which passes into
the mtercellular system of spaces, is liable to be rapidly removed under the iufluence
of ccntmued active transph-ation, and this, under the circumstances, implies an absolute
diinmutiou m the entire stock available for the up-kcep of turgeseence. The tissues
winch prmiarily lost turgcscenee in connection with increased filtration remain as
active as ever. They are just as eager ,to satisfy their osniotie capacities as they
were before, and m then- efforts to do so they must tend to interfere with ti»
supply of neighbounng tissues jiist in proportion to the amount of actual loss in the
general stock of water which was induced by the primary filtrative discharge. The
spread of movements along the com-se of a rachis must certainly be favoured by such
a distuAance m tlie supply of water; and, as in normal leaves it is very difficult to
secure tba any mechanical disturbance has been primarily strictly localised to one
or other of the mdrvidaal pinnules, it is little wonder that, in the presence of external
conaitions implying maximal instability of position, cortain extensiou of luoveiiient
should occur even in cases in which the direct mechanical action of the moving pinnules
upon then- neighbours cannot fully acconnt for the phenomenon. In any case
ments following contact have a very limited extension and, under circuLtances in wHch
the evidence of other so-called stimuli is followed by more or less extensive propagation
1 normally strictly localised to the points primarily affected. They
.necessarily involve any actual loss of liquid, but mere temporaiw
nd hcnce they are not iu themselres efficient causes of widely
of supply and loss of liquid in the actively turgeseent
of inovcuient, they
Jo not in themselvi
redistribution of it,
diffused disturbance
This is no longer the case when the primary agent in determining the occurrenca
of movement is a rupture in the continuity of the tissues. Incision of the a.xis or petiole
or amputation of portions of the kmrn® of pinnuks, is followed not merely by movements