114 THE CAUSES OF FLFCTtJATIO::iS IN TUEGESCEXCE
tlie couditions arc such as necessarily to imply increased iu place of decreased watersupply.
No theory of tho comraunication of impulses giving rise to active protoplasmic
coutrivction in the motor organs will account for these pheuoDieua, becausc none of tiiem
can account for tho fact that the propagation of movement should, in the case of young
shoots, occur in both directions and iu that of old ones in one only, or should, at all
events, not extend beyond the immediate neighbourhood of tho site of application, or,
iu other words, bo confined solely to the area in which obstruction can como into
play in inducing decreased supply of water. The theory of active protoplasmic
conti-action here once more breaks down, whilst the i)liysical theory is able to afford a
satisfactory explanation.
Tho phenomena of distribution of movement following the local application of heat
to some point in the course of a mature axis of Mimosa pudica may be reproduced in a
suitably arranged picce of apparatus (Plate I, Fig. 1). This consists of a stout, upright
metal tube closed at the ujiper extremity, provided with a number of slender, short
lateral branches, and with a closely fitting stop-ccek inserted at such a point in its
length that some of the branches He to its proximal and others to its distal side. The
branches ai-e open at their extremities, and can readily be connected with suitably weighted
pieces of glass-tubing by flexible joints of rubber-tubing. The majority of the glass
tubes are sealed at the end, but one or more of them are di-awn out into fine capillary
ends and left open. Their weight ought to be such as to secure a certaiu amount of
flexion of the rubber-joints when tlie apparatus is filled with water. If the lower end
of the main tube be connected with a stream of water under a sufficiently high pressure
and the stop-cock be left open, fine jets of water will, of course, escape continuously from
the capillary extremities of the open tubes, and the flexion of all tho rabber-joints will
diminish as pressure rises, with corresponding erection of the glass tubes connected witli
tlicm. Eut, on sudden closure of the stop-cock, escape of water will continue to occur from
the capillary orifices for some time, and with this the rubber-joints will recur to their
origiual state of flexion in all the branches situated to the distal side of the stop-cock,
whilst all those on the proximal side will retain their position of erection unaltered.
Here, then, we have a distribution of movement exactly parallel to that occurring in the
case of matui-e axes of Mimosa on local application of heat at some point in their course.
The distal discharge of water from the capillary orifices represents transpiratory loss,
tho stream of water entering the lower extremity of tho main tube represents rootsupply,
and the rubber-joints represent portions of tissue, like those present in the
motor organs of the leaTes, wliich are liable to undergo rapid alterations iu strength in
connection with alterations in the relations between general supply and loss of water.
Tho sudden closure of the stop-cock suddenly obstructs all further access of water to
the distal part of the apparatus, and as distal loss continues, a rapid fall in pressui-e
takes place throughout the whole distal parts of the apparatus, the rubber joints cease to
be turgescent, and the position of the glass tubes comes to be that which in determined
by mere passive structural properties. But in the proximal port of tlie apparatus the
conditions of water-supply and loss remain unaltered, or, if anything, a rise in pressure
takes place, and hence there is no weakening of the joints or depression of tho branches.
In those cases in which centripetal movements occur in connection with the application
of heat to mature axes of Mimosa pudica, these movements are no doubt dependent on
centripetal extension of the obstmctive gaseous expansion, so far as to involve the portions
IN THE MOTOR ORGANS OF LEAVES. 115
of the water-conducting system concerned in the supply of the leaf which moves. They
occur only when the point of application is immediately above tho site of origin of
a petiole, and they never extend beyond that leaf.
Not only are the effects which are produced by so-called " stimuli " in giving
rise to movements in the leaves of Mimosa pudica profoundly modified by the presence
of external conditions affecting supply and loss of water, but the effects which
follow the action of individual " stimuli " differ in connection with the degree to
which the latter are of a nature to give rise to disturbances in tho relations of
general supply and loss. At any given period it will be found that simple contactdisturbance
gives rise to less effect thaii section or other interruption in the continuity
of the tissues does, and that the latter, in its tuni, is less efficient in giving rise
to movement than tlie application of strong heat is.
When soil and air are alike humid, contact-" stimulation" occasions pm-oly localised
effects. If pressure be applied to the under surface of the primary pulvinus or the
upper sui-face of the primary petiole, full depression of the latter takes place, but the
distal portions of the leaf show not the faintest traces of movement, the secondary
rachises remaining fully divergent, and the pinnules in tho maximal diurnal position
of expansion. Under similar circumstances the movements of individual pinnules
which have been subjected to contact are equally purely localised, so long as displacement
of one does not imply mechanical disturbance of another. The movements
of individual terminal piuîmles are, of course, not necessarily liable to give rise to
any considerable mechanical disturbance elsewhere, but, in the case of all tho others,
a disturbing influence is introduced by the fact that they are ordinarily set upon thé
radiis at such narrow intervals that, when in the diurnal position, the posterior half
of each lamina overlaps the anterior half of the lamina of the pinnule situated
behind it on the rachis (Plate V, Fig. 3). Owing to this arrangement, the sudden
elevation of any pinnule must necessarily give rise to a certain amount of upward
pressure on the one in front of it. In spite of this, however, purely localised
pinnular movements may very readily be evoked even in perfectly normal leaves
when couditions of telluric and atmospheric moisture are very high. Where couditions
of moisture are not so excessive, movements in normal leaves are liable to extend
beyond tho primarily affected pinnule for a certain distance along the rachis in a
centrifugal direction. Under these cii-cumstances, in ozxJer to obtain perfectly localised
pinnular movements, it is necessary to select particular leaves for expcziment. It is
not at all unusual to meet with leaves which deviate so far from tho normal standard
as to have their pinnules set on so far apart that, when in their maximal diui-nal
position, they do not overlap one another, but stand quite free nnd apart as those of
Leucaena glanca normally do. In such leaves it is quite easy to show that, even
when couditions of humidity are only moderately high, pinnular movoments evoked
by contact are purely localised. In sncli cases it is easy to cause full displacement
of alternate pinnules all along one or both sides ot the rachis wilhont inducing the
slightest movomont in tlie intermediate ones wMch are not directly subjected to
contact. The displacement may be causnd either by pressure on the upper surface
of the tertiary pulvinus or on the under surface of the lamina: iu both cases alike
the moyemeut remains strictly localised. When atmospheric and tcllmie liumidity